Introduction

Early diagnosis of pregnancy following assisted reproductive technology cycles is critical both for the emotional well-being of the couple as well as for management guidance for their health care providers. While transvaginal ultrasound is a useful tool in diagnosis of early pregnancies, it is typically non-diagnostic before 5–6-week gestation. Serial human chorionic gonadotropin (hCG) measurements in early pregnancy can allow for diagnosis, monitoring, and potential early differentiation of normal versus abnormal pregnancies. Ideally, this would lead to reduced stress surrounding uncertainty of treatment success and to swift management to avoid delaying further attempts at conception if desired.

Fertility clinics may follow different protocols, but initial serum hCG measurement typically occurs around 11 days following transfer of cleavage stage embryos or 9 days following transfer of blastocysts, a time that correlates with the 14th day following oocyte retrieval. Serum hCG can then be followed prior to scheduling an ultrasound. In spontaneously conceived normal pregnancies, the rate of hCG rise has been reported to be at least 53% in 2 days, with a median increase of 50% in 1 day and 124% in 2 days [1]. In order to characterize the rate of hCG rise for pregnancies conceived through IVF, a retrospective cohort study in 2005 demonstrated that the average rate of rise in pregnancies that resulted in live birth was 50% in 1 day and 124% in 2 days, similar to spontaneous pregnancies [2]. There was, however, an earlier plateau in IVF pregnancies (38 gestational days at concentration of approximately 3000 mIU/mL) than in non-IVF pregnancies. The authors reported the slowest confirmed rise for a normal pregnancy was 30% in 2 days.

As a normal rise is predicted in hCG levels following in vitro fertilization cycles, it is possible to identify those pregnancies that may be abnormal earlier than otherwise detected by ultrasound by using the initial hCG level or by following the hCG trend. Abnormal levels could occur in biochemical pregnancies (15%) [3], nonviable pregnancies, ectopic pregnancies (1.6%) [4], and in other clinical scenarios. There may, in fact, be factors related to the IVF techniques themselves or the infertile patient population that leads to alterations of hCG dynamics. This paper aims to provide a review of the potential alternate causes of abnormal hCG rise following assisted reproductive technology.

Methods

A review of the published literature was done to find the role of early hCG monitoring on pregnancy outcomes in assisted reproductive technology (ART) treatment cycles (Table 1). A literature search was performed using PUBMED and Google Scholar to find articles about hCG levels following assisted reproductive technology treatments.

Table 1 Published literature of early serum hCG levels following IVF
Full size table

The search was conducted using the following keywords: “hCG,” “human chorionic gonadotropin”; β-hCG,” “assisted reproductive technology”; “IVF,” “in vitro fertilization”; “ICSI,” “intracytoplasmic sperm injection”; “single-embryo transfer”; “multiple embryo transfer”; “embryo morphology”; “singleton”; “multiple gestation”; “twin gestation”; “vanishing twin”; “pregnancy outcome”; “live birth”; “maternal age”; “aneuploidy”; “trophoblast”; “embryo stage”; “cleavage stage”; “blastocyst”; “embryo biopsy”; “trophectoderm biopsy”; and “PGT.” The abstract and purpose of the articles were reviewed, and the publications selected included the abstracts, original articles, case reports, cohorts, or case series published between 1980 until time of publication which discussed the hCG levels and pregnancy outcomes following ART. Only articles written in English were included.

hCG Levels and Pregnancy Outcomes

The most clinically important motivation for evaluating early hCG levels following IVF pregnancies is to predict pregnancy outcomes. Either a single initial hCG value or an hCG rise that is indicative of normal vs. abnormal pregnancy outcomes would be of great interest in counseling after pregnancy is achieved through IVF.

An analysis of 6021 pregnancies published in 2009 characterized the initial hCG level and typical rise of serum hCG levels that correlated with live birth following in vitro fertilization [6]. These authors found that initial hCG was predictive for delivery rates both for the population as a whole and when broken down by age group. The percent rise over 2 days was added to the predictive value, and it was found that delivery rate increased progressively as the percent rise went from 0 to 100%, with no increased chance of delivery over 100% rise. The lowest initial level of hCG that resulted in live birth was 5 mIU/mL.

Another study published in 2007 looked specifically at outcomes of pregnancies with low initial hCG level, which they considered to be < 150 mIU/mL on day 13 after embryo transfer [7]. This group found that an initial hCG level < 150 mIU/mL was correlated with unfavorable outcomes (biochemical or ectopic pregnancy or spontaneous abortion), and this persisted across treatment protocol, fertilization method, and type of embryo transfer (frozen or fresh). Likelihood of delivery dropped from 77.8% in patients with an initial hCG > 150 mIU/mL to 35.2% in those with hCG < 150 mIU/mL. Of those with an initial low hCG level that reached delivery, there were no increased adverse outcomes.

Many investigators have sought to determine a “cutoff” value to predict type of clinical pregnancy following in vitro fertilization. A retrospective data analysis published in 2014 analyzed hCG values on day 12 after embryo transfer of 1057 clinical pregnancies and performed receiver operating characteristic curves to determine optimal cutoffs discriminating between singleton and multiple gestation, intrauterine and ectopic pregnancy, and miscarriage and live birth [8]. They determined a cutoff value of 239 IU/L as 69% sensitive and 74.5% specific for multiple pregnancy. For ectopic pregnancy they found a level of 91 IU/L would have a negative predictive value of 98.5%, and for miscarriage a level of 143 IU/L would have a negative predictive value of 90%.

More recently another retrospective cohort investigated initial hCG level and rate of increase to predict pregnancy outcome in hopes to determine a cutoff value or rate of rise for biochemical pregnancy, early pregnancy loss, and live birth of singleton pregnancy [9]. This group found the hCG level measured 14-day post-oocyte retrieval to be the most sensitive with the highest positive predictive value. Specific cutoffs to define biochemical or early pregnancy loss were not calculated, but mean day 14 hCG level for biochemical pregnancy was 38.6 mIU/mL, and for early pregnancy loss was 137.5 mIU/mL. Mean day 14 hCG level for live birth was 197.2 mIU/mL. Receiver-operating characteristic (ROC) curves were performed to determine a cutoff value to predict live birth. In fresh cycles a level of 104.5 mIU/mL was 80.3% sensitive for live birth, with a positive predictive value of 80.8%. In addition, adding the rate of rise improved predictability, with a fold change of 2.78 having a 70.6% positive predictive value for live birth.

Abnormal hCG Levels in Single-Embryo Transfer Cycles

The studies discussed above did not standardize the number of embryos transferred, and many patients included in these studies received multiple embryos. To avoid the potential confounding impact of transfer of multiple embryos on hCG levels, a review of hCG trends following single-embryo transfer is warranted. Al Mamari et al. investigated initial hCG level in a total of 1076 pregnancies resulting from single-embryo transfer between 2013 and 2017 [10]. In those pregnancies with an initial day 16 hCG level of 103 ± 13 IU/L, 50% had a biochemical loss. Biochemical pregnancies were detected in 21% of patients at levels from 136 to 197 IU/L, and 12% at levels 199–252 IU/L. They determined that a cut-off for clinical pregnancy was 190 IU/L, and for live birth was 213 IU/L. While this study did not examine the early hCG trends, this is certainly valuable information in discussing risk of biochemical pregnancy with patients with initial low level hCG.

Mean initial hCG levels were also evaluated on post transfer day 12 in a 2004 study of 370 consecutive single top-quality day 2 or 3 embryo transfers [11]. One hundred and ninety-two pregnancies resulted from these cycles, with 30 biochemical pregnancies, 4 ectopic pregnancies, 23 miscarriages, and 135 ongoing pregnancies at 12-week gestation. A cut-off hCG value of > 45 IU/L on day 12 post transfer was determined by ROC curves to have a 75.6% sensitivity for ongoing pregnancy. This group did evaluate rate of rise of hCG, but over 4 days rather than the 2-day standard.

A 2009 study of 230 pregnancies resulting from single blastocyst transfer evaluated hCG levels on day 11, 14, and 21 following oocyte retrieval [12]. This study did not evaluate the rate of rise of hCG. Pregnancies with multiple gestation were excluded by only including those pregnancies with a single gestational sac on day 21. On day 14, mean hCG levels were 73.1 mIU/mL in the miscarriage group vs. 141.7 mIU/mL in the term delivery group. The day 21 hCG value gave the best prediction of potential viability when ROC curves were used, with mean hCG levels for the miscarriage group being 1075.0 mIU/mL vs. 2946.0 mIU/mL in the term delivery group. They determined cut-off values for probability of live birth of 80 mIU/mL and 1500 mIU/mL on days 14 and 21, respectively, with positive predictive value of 93.3% and 91.7%.

Transfer of Multiple Embryos Resulting in Singleton Pregnancy

Often in assisted reproductive technology cycles the recommendation is made for transfer of more than a single embryo to increase the likelihood of pregnancy based on female age, embryo quality, history of IVF failure, or some other prognostic factor. It is plausible that patients who have more than one embryo transferred may have transient implantation of nonviable embryos that could contribute to hCG concentration without ever developing ultrasound evidence of multiple pregnancy. This could very likely lead to an hCG trend that appears abnormal.

A recent retrospective cohort was designed to investigate this scenario, and results revealed that 6.1% of patients who received two or more embryos compared with only 2.7% of patients with single-embryo transfer had an abnormal hCG rise (defined as < 66%) between the first and second measurements [13]. In those patients that had hCG values that initially rose abnormally, three quarters then had a normal rise between the second and third measurements.

This same group had previously investigated the hCG rise in IVF pregnancies with a vanishing twin (two gestational sacs detected on early ultrasound but subsequent failure of one twin prior to 12-week gestation). This prior data revealed that pregnancies with a vanishing twin did have a slower initial increase when compared with normally progressing singleton or twin pregnancies (114.3% vs. 128.8% and 125.4%, respectively); however, the rise was still within clinically accepted limits [14].

The phenomenon of vanishing twin is important prognostically for a couple who has achieved pregnancy following IVF. Almog et al. demonstrated worse obstetric outcomes for vanishing twins that resulted from IVF compared with singleton pregnancies resulting from IVF, with increased risk of preterm deliveries and low birth weight in these pregnancies [15]. Additionally, Márton et al. demonstrated worse perinatal outcomes for patients with vanishing twin pregnancies conceived following IVF when compared with spontaneously conceived vanishing twin [16]. These worsened outcomes included growth restriction, placental abruption, and retained placenta. These authors proposed that this could be related to the greater gestational age that vanishing twin is detected in IVF pregnancies compared with spontaneous pregnancies [17] and that later timing of demise would tend to indicate that there is impaired uterine environment or impaired utero-fetal interaction which could subsequently lead to the above mentioned complications.

Multiple Gestation

Given what we know about hCG levels and trends in healthy singleton pregnancies, it may be postulated that multiple gestation resulting from assisted reproductive technology may contribute to an hCG rise that could appear abnormal as there are two (or more) embryos and therefore additional sources of hCG. Perhaps the presence of multiple embryos would lead to a higher initial hCG level, and a steeper rise in hCG levels.

When studied, this has not been observed in multiple gestations resulting from assisted reproductive technology. The previously discussed study that investigated hCG patterns in ART pregnancies did compare trends between singleton, twin, and triplet gestations [2]. The findings revealed that while Initial HCG values were significantly higher for twin and triplet gestations compared with singletons, the rate of rise was unchanged.

Outcomes from 177 IVF pregnancies between 2009 and 2014 were analyzed in a recent retrospective study to determine the hCG value at day 13 after embryo transfer that could predict live birth in singleton and twin pregnancies [18]. Fifty singleton births, 50 twin births, 27 vanishing twin pregnancies, 43 first trimester singleton losses, and 7 first trimester twin losses were included. In singleton pregnancies, a day 13 hCG level of < 85 mIU/mL had an 89% risk of first trimester loss, whereas a concentration > 386 mIU/mL had a 91% chance of live birth. In twin pregnancies, a concentration of < 207 mIU/mL had only a 33% chance of delivery of twins vs. 55% chance of vanishing twin, compared with a level of > 768 mIU/mL which was associated with an 81% chance of live twin birth.

Embryo Stage on Day of Transfer

Embryo transfer following IVF can occur at the cleavage stage (2–3 days after fertilization), or the blastocyst stage (5–6 days after fertilization). The question arises whether transfer at different developmental stage has an impact on early hCG levels and pregnancy outcome. If hCG levels are measured at equivalent time points from oocyte retrieval, there would not be expected to be a significant difference in the two groups as long as embryos progress and develop as expected, unless perhaps there was an impact of longer time spent in vitro culture.

A retrospective cohort study of 560 women with clinical pregnancy following embryo transfer evaluated hCG level following day 3 and day 5 transfers, without demonstrating a significant difference between groups [19]. Initial hCG levels were 387 IU/L for day 3 embryos and 352 IU/L for day 5 embryos on the same day post retrieval. This lack of difference persisted even when subdividing into singleton vs. multiple gestation, although numbers of multiple gestation were small. There were only a limited number of patients in the blastocyst transfer group (only 60 compared with 500 cleavage stage) in this study which may limit the findings. It is possible that the statistically insignificant difference could be due to the fact that embryos develop faster in vivo than in vitro. The cleavage stage embryos transferred perhaps implant and start producing hCG sooner (even by few hours) as compared with the blastocysts transferred.

A larger retrospective cohort including 2035 embryo transfer cycles (1422 on day 3, 613 on day 5) investigated the potential difference in initial hCG and pregnancy outcome [20]. These researchers found an almost two fold increase in hCG values in day 5 transfers compared with day 3 transfers. However, in this study, hCG level was checked 12 days following transfer in both groups rather than at the same day post retrieval/fertilization and so this difference is not unexpected as the embryos would be at different developmental stages and therefore the length between implantation and hCG testing differed.

In a study that compared levels at the same time post-fertilization, hCG levels were actually significantly lower (102 IU/mL vs. 155 IU/mL) in day 5 embryo transfers compared with day 3 [21]. The authors proposed three possible mechanisms for this finding: first being a difference in very early, subclinical losses; second that delaying to day 5 leads to a gender bias which could relate to rate of embryo development; and third being that the additional 2 days in culture may adversely affect the development of the embryo leading to a subtle delay in hCG production. They went on to explain that their findings were inconsistent with the first two possibilities and concluded that it is most likely related to a potential impact of additional time in culture. We believe it is likely that additional time in culture results in lagging development as discussed above, with potential for these embryos to implant and begin producing hCG later.

Embryo Morphology at Transfer

Embryos are graded with a morphology system that considers number and symmetry of cells and degree of fragmentation for cleavage stage embryos, and expansion and quality of the inner cell mass and trophectoderm for blastocyst stage embryos. As mentioned earlier, studies have compared hCG levels after cleavage stage or blastocyst transfer, with inconsistent results, but only a handful of studies include the impact of morphology.

A recent retrospective cohort was performed with the primary goal of the effect of embryo morphology on serum hCG levels and subsequent live birth, and a secondary goal of determining effect of cleavage vs. blastocyst stage embryo on hCG level and live birth [22]. It included 455 patients who underwent fresh single-embryo transfer cycles between 2011 and 2016 which resulted in 142 patients with positive hCG levels 12 days after transfer, 85 of whom had a documented clinical pregnancy which resulted in 79 live births. The mean initial hCG level for cleavage stage transfer and blastocyst transfer was similar (315 vs. 372 IU/L). For cleavage stage embryo transfers, which were graded by cell number, blastomere size, and fragmentation rate, there was no difference between morphology and hCG resulting as positive or negative on day 12. For blastocysts, which were graded by level of expansion and grade of inner cell mass and trophectoderm, trophectoderm cell number, and blastocyst quality score, more expanded embryos and those with a higher blastocyst quality score were more likely to result in a positive hCG level. Interestingly, blastocyst morphology was shown to have an effect on serum hCG levels in cycles that resulted in ongoing pregnancy and live birth, with blastocyst expansion, trophectoderm cell number, and blastocyst quality score playing a role. However, in this study of relatively young patients (mean age of 30) there was a lower than expected clinical pregnancy and higher than expected biochemical pregnancy rate, so the findings should perhaps be revisited.

A French study published in 2018 aimed to evaluate the association between blastocyst morphology and early hCG rise to further investigate any possible correlation [23]. In this retrospective cohort study, the rate of hCG rise was calculated in 455 single-blastocyst transfer cycles that resulted in positive pregnancy tests. This study revealed that while there was a significant difference attributed to blastocyst expansion, there was considerable overlap between groups (blastocyst quality B1 through B5), and the rate of rise was more affected by ultimate pregnancy outcome.

hCG Levels Following Fresh Vs Frozen Embryo Transfer

Another factor that may potentially impact hCG dynamics in early pregnancy could be whether pregnancy had resulted from a fresh or frozen embryo transfer, and further what technique (vitrification vs. slow freezing) had been used for embryo cryopreservation. Much research has been done regarding the effects of cryopreservation on embryo developmental potential which could be reflected initially by its ability to develop to blastocyst, implant, and produce hCG. For example, lower levels of hCG may reflect a compromised trophoblast.

Sites et al. reviewed their own clinic’s single-embryo transfer cycles from 2007 to 2012 to evaluate whether there was an impact of cryopreservation on hCG dynamics and pregnancy outcomes [24]. During this time period they initially used a slow freezing technique and then as technology evolved began using vitrification for cryopreservation of supernumerary embryos, so they have data on each method. They discovered that there was a significantly lower median initial serum hCG level in thawed-warmed cycles compared with fresh cycles (100 vs. 126 mIU/mL). Additionally, a significant difference was noted between slow freezing and vitrification methods, with median initial hCG levels of 89 and 183 mIU/mL, respectively. Despite the difference in initial hCG levels, live birth rates were similar between fresh transfers and most methods of embryo cryopreservation other than blastocyst slow freezing (which had the lowest initial hCG levels). This study was limited by small numbers in some of the groups evaluated as they had changed their methods several times over the study period and included both cleavage and blastocyst stage.

A more recent study of 1130 single blastocyst transfer cycles (789 fresh and 341 vitrified-warmed transfers) found that while overall initial hCG levels were comparable between groups, those pregnancies that resulted in clinical pregnancy had a significantly higher initial hCG level after the transfer of vitrified embryos vs. fresh transfers (383 vs. 334 IU/L), with a higher threshold value for predicting clinical pregnancy required of vitrified embryos (137 vs. 111 IU/L) [25]. These investigators concluded that each clinic should determine its own threshold but should consider that higher levels of hCG may be expected after a vitrified-warmed transfer cycle. Conversely, a similar study of blastocyst transfer after fresh or vitrified-warmed cycles demonstrated no difference between initial hCG level and type of cycle, although hCG levels in those that resulted in live birth were still significantly higher than non-viable pregnancies for both groups [26].

In a study evaluating the impact of vitrification vs. slow freezing on day 3 embryo transfer cycles, the method for cryopreservation did reveal a significant difference in initial hCG levels [27]. These investigators found that while pregnancy outcomes did not differ between groups, initial hCG levels for ongoing pregnancies were significantly lower for vitrified-warmed cleavage stage embryos than slow frozen embryos (279 vs. 320 IU/L).

Intracytoplasmic Sperm Injection Versus Conventional Fertilization Methods

Given the additional micromanipulation of oocytes that is required during cycles that use intracytoplasmic sperm injection (ICSI) for fertilization, it could be predicted that ICSI could have an effect on early pregnancy dynamics. This additional manipulation had earlier been postulated to potentially lead to possible delayed implantation of the embryo, or even increased degree of embryo fragmentation, which could impact early hCG levels [28, 29].

A retrospective analysis of 204 pregnancies resulting from in vitro fertilization cycles compared initial serum hCG levels of singleton pregnancies resulting either from ICSI or conventional fertilization (IVF) [28]. These authors found no significant difference in hCG levels on day 14 or 16 post embryo transfer, and no difference in the rise between day 14 and 16 whether conventional IVF or ICSI was used.

Another retrospective analysis investigated whether there was an effect from ICSI on twin pregnancies [30]. This group found that there was a significantly higher initial hCG in twin pregnancies resulting from ICSI compared with conventional fertilization, but this effect did not persist when controlled for confounding factors.

Embryo Biopsy for Preimplantation Genetic Testing

Preimplantation genetic testing (PGT) is accomplished through biopsy of the embryo and evaluation of the removed cells for genetic makeup. Given the additional manipulation with this procedure and the physical removal of either blastomere at eight cell stage or trophectoderm cells, it is plausible that there could be decreased initial hCG levels as an effect of a lag in development to blastocyst or decreased trophectoderm cell numbers, respectively. Data are limited but there have been some investigations into early hCG levels following cycles with embryo biopsy for PGT.

In 2011 Cho et al. published their data comparing initial serum hCG levels in 129 pregnancies following PGT with 1161 age-matched controls who underwent ICSI cycles [31]. Their data revealed that mean hCG level was consistently lower for those patients who had PGT: 37.9 mIU/mL vs. 46.9 mIU/mL on day 12 post retrieval, 111.3 mIU/mL vs. 145.4 mIU/mL on day 14 post retrieval, and 2225 mIU/mL vs 3614 mIU/mL on day 21 post retrieval. The doubling times were not significantly different. Despite the lower initial hCG levels, there was no significant difference in biochemical pregnancy rate, miscarriage rate, second trimester losses, preterm delivery rates, or birth weights between groups.

More recently, trophectoderm biopsy has become the standard method for PGT; however, data on the impact of this procedure on initial hCG levels is limited. Soriano et al. recently presented their data on the effect of trophectoderm biopsy on early serum hCG levels in pregnancies that resulted in singleton live birth [32]. Two hundred-seventy-nine PGT cycles were compared with 208 cycles without trophectoderm biopsy, and there was no significant difference noted in mean initial or subsequent hCG levels. There was a statistically significant difference in the rate of rise (247.9% vs. 238.9% in the PGT vs. non-PGT groups respectively); however, this difference may be clinically less significant.

Ovarian Hyperstimulation Syndrome

Ovarian hyperstimulation syndrome (OHSS) is a complication of controlled ovarian stimulation that involves potentially massive fluid shifts from intra to extravascular compartments. Given these fluid shifts, it is reasonable to consider the possibility that serum hCG levels may appear abnormal in pregnancies resulting from in vitro fertilization cycles that were complicated by OHSS.

A recent case control study did demonstrate altered early hCG trends in pregnancies complicated by OHSS without change to obstetrical or neonatal outcomes [33]. This study compared 77 pregnancies complicated by OHSS with 231 IVF-conceived pregnancies without OHSS which resulted in 69 deliveries in the OHSS group and 160 in the control group. At all time-points hCG values were lower in OHSS pregnancies than in controls, as was the relative change of hCG values between time points. Interestingly, in women who underwent paracentesis therapy for OHSS, rate of hCG rise increased following the procedure but then fell again several days following treatment (potentially indicating re-accumulation of ascites). Importantly, pregnancy outcomes remained the same despite the altered hCG kinetics.

Similarly, a case report that was recently published found a similar pattern in a patient suffering from OHSS following a gonadotropin cycle with intrauterine insemination [34]. hCG level in the first interval only rose 23%, and in the next time interval only 29%. As symptoms resolved, hCG trend appeared to normalize, and ultrasound revealed a normal intrauterine pregnancy at 8-week gestation. Delivery information for this patient was not available at time of publication.

Aneuploidy

Prenatal aneuploidy screening is offered to most pregnant women and various types of screening tests are available at different gestational ages [35]. First trimester screening includes a single maternal serum hCG and pregnancy-associated plasma protein A (PAPP-A) level and nuchal translucency on ultrasound and is the earliest option offered from 11- to 14-week gestation. hCG values are also used in second trimester screening options. While these aneuploidy screening options do include a single hCG level in conjunction with other markers in determining risk of chromosomal abnormalities, information about early hCG rise in pregnancies that result in chromosomally abnormal pregnancies is limited. However, if there were a correlation, this would certainly be good information to have to prompt appropriate counseling and earlier definitive screening when available.

A prospective study between 1984 and 1990 of pregnancies conceived through infertility treatment evaluated hCG levels in early pregnancy and reported subsequent chromosomal abnormalities [36]. Of 573 patients, 15 had first trimester losses with confirmed abnormal karyotype. A significant difference in hCG measurements was noted between normal and chromosomally abnormal conceptions. Normal pregnancies from both natural and stimulated cycles demonstrated a linear and then curvilinear progression of hCG until a plateau of approximately 110,000 mIU/mL at 10-week gestation, whereas chromosomally abnormal pregnancies conceived spontaneously remained linear with a max hCG concentration of 10,000 mIU/mL at 7-week gestation, followed by decline until spontaneous abortion. Similarly, for those women who conceived chromosomally abnormal pregnancies through infertility treatment, hCG concentration rose to a max of 20,000 mIU/mL and then declined. This suggests a difference in the rate of hCG rise for chromosomally abnormal pregnancies whether spontaneous or conceived through IVF.

Placental Abnormalities

hCG is produced by the synctiotrophoblast following implantation, and it follows therefore that if hCG secretion is abnormal, perhaps there is a higher likelihood of placental abnormalities in these pregnancies. In regard to the possibility of trophoblast dysfunction marked by abnormal hCG levels, we can look to early pregnancies with abnormal hCG and subsequent poor pregnancy outcomes.

Johnson et al. followed women who conceived following IVF with weekly serum analytes and ultrasound to characterize the endocrine changes in early pregnancies that result in miscarriage following detection of fetal cardiac activity [37]. Four women were found to have a miscarriage on ultrasound following initial documentation of fetal cardiac activity between 9- and 12-week gestation, and seven had losses between 16- and 20-week gestation. Of the four with earlier losses, three had hCG levels consistently beneath the 10th percentile and all had decrease in serum levels prior to loss of cardiac activity. In the women with later losses, placental protein levels including hCG were reduced several weeks before fetal demise occurred while fetal growth remained intact, suggesting the trophoblast was functioning abnormally prior to the effects on the fetus were seen.

Maternal Factors and hCG Levels After IVF

Age

As more women are delaying childbearing and age-related declines in fertility occur, ART has been increasingly used for this older population, and there is a well-established effect of maternal age on pregnancy outcome. Therefore, interpretation and counseling regarding an initial hCG level should likely be considered in relation to age.

The previously discussed study by Shamonki et al. found that maternal age was an important variable when assessing initial hCG level following IVF [6]. As age increased there was a significant reduction in live delivery rate related to age for a given initial hCG level, and a second hCG level drawn 2 days later added predictive value. This indicates that age should be considered when counseling a patient following their first hCG test.

In 2017 a multicenter cohort study investigating the influence of female age on oocyte, zygote, and embryo morphology in addition to cycle outcome was published [38]. This study included data from 11,744 IVF/ICSI cycles and evaluated initial hCG level in a post-hoc analysis. The initial hCG value (14 days after day-2 embryo transfer) that resulted in ongoing pregnancy was increased significantly with increasing age of the woman in both IVF (429 cycles) and ICSI (374 cycles) groups. This would seem to echo the previously discussed study as age increases a higher initial hCG level would be required for reassurance that pregnancy would be ongoing [6].

It seems that patients with late maternal age deserve specific consideration as outcomes may be more pressing for this group as time may be limited for this population. They are also known to have poorer outcomes with fertility treatment and any early reassurance that could be provided would be beneficial.

Body Mass Index

Two recent retrospective cohort analyses were designed to investigate the association of body mass index (BMI) with serum hCG levels and rate of hCG increase [5, 39]. The results revealed that while the rate of increase in β-hCG was unaffected by body weight, the initial serum hCG values were significantly negatively correlated with BMI. Since increasing body weight is associated with significantly lower β-hCG, it is plausible that the initial hCG values lacks sensitivity for pregnancy outcome in patients with obesity.

Discussion

The waiting period between embryo transfer and pregnancy test has been shown to be a period of increasing anxiety and depression, with anxiety persisting even in women whose pregnancy test came back positive [40]. Fertility providers may also be emotionally invested in their patients’ outcomes, and in addition would benefit from early determination of pregnancy potential in order to counsel patients, manage abnormal pregnancies, and proceed with additional treatment if desired in a timely manner. The earliest test for whether a cycle is successful is the initial hCG value, and this value is typically repeated over a number of days to help provide some insight into whether a pregnancy may be normal or abnormal. Despite this early indication, there are no accepted “cutoff” initial levels or rates of increase that can definitively diagnose pregnancies that will be abnormal and those that will result in live birth, so uncertainty remains. Additionally, the different factors and technologies involved in IVF further complicate the matter, and hCG levels are not necessarily constant across different labs and measuring platforms, which limits universalization. Ultimately, ultrasound still must be performed, but in the early days prior to ultrasound ability to detect a clinical pregnancy, knowledge of normal hCG characteristics may reassure a patient. The aim of this review was to evaluate the current literature in regard to initial hCG values and trends in IVF patients, in order to provide earlier counseling and guidance.

While there will likely never be a single hCG level or rise in hCG that can be entirely reassuring for successful pregnancy following IVF, the review of the above studies hopefully does provide some information as to how a fertility specialist can begin to counsel their patients during the very early stages of pregnancy, and how to identify patients that may need closer monitoring, earlier ultrasounds, or need preparation that their pregnancy may not be normal. It also demonstrates that there can be a wide range of normal initial values that do not necessarily lead to poor outcomes and that alone can provide some reassurance to patients in the uncertain window between embryo transfer, pregnancy test and ultrasound. Repeated, larger studies are required to study these factors in further depth and to provide more support to the trends, although a framework for counseling can be developed based on the information that is currently available.