Introduction

Emergencies during the first trimester of pregnancy are common, with women presenting with abdominal pain and vaginal bleeding representing more than 1% of all visits to the emergency department (ED) [1]. These patients can have a wide spectrum of pathology, ranging from more benign etiologies such as hemorrhagic ovarian cysts and subchorionic hematomas, to potentially life-threatening conditions such as ruptured ectopic pregnancy. Accurate diagnosis of women with common first-trimester emergencies such as ectopic pregnancies is challenging, as more than 50% of women are asymptomatic before ectopic pregnancy rupture [2] and over 40% of women with ectopic pregnancy are misdiagnosed with less severe complications of early pregnancy or non-pregnancy-related conditions [3,4,5].

It is imperative for emergency radiologists to be familiar with the current literature, clinical practices, and terminology of first-trimester complications. This will not only allow efficient and safe evaluation of patients with symptomatic first-trimester pregnancy, but also allow them to provide their obstetrical (OB) colleagues with the information needed to make clinical management decisions. The purpose of this article is to review the development of the early embryo, present the common causes and imaging appearance of first-trimester emergencies, and discuss key management pathways, so that radiologists can facilitate accurate and timely diagnosis and help avoid potentially serious complications.

Development of the early embryo

Development of the early embryo is a stepwise process that begins with ovulation. Ovulation, the release of an oocyte from a dominant ovarian follicle, is initiated by a surge in luteinizing hormone and on average follows the last day of the previous menstrual cycle by 14–21 days [6, 7]. Once released, the oocyte traverses the fallopian tube, a process that takes approximately 24 h. If a single sperm fertilizes the oocyte, conception occurs. The conceptus will then proceed to the uterus and implant within the uterine wall approximately 8–10 days after ovulation in most successful pregnancies. The early embryo divides rapidly and begins to make human chorionic gonadotropin (hCG), which contributes to progesterone production and maintenance of the corpus luteum.

At 5 weeks gestational age (GA), or 3 weeks post-ovulation, a gestational sac is generally visible by transvaginal ultrasound, appearing as an anechoic fluid collection embedded within the endometrium (Fig. 1a). It is important to note that, in a woman with a positive hCG, any intrauterine fluid collection is highly likely to be a gestational sac [8]. By 5.5 weeks GA, the yolk sac should be visible. The purpose of the yolk sac is to provide nutrients to the developing embryo, participate in early hematopoiesis, and contribute to metabolic and immunologic functions. The yolk sac appears as a well thin-walled, circular structure with an anechoic center (Fig. 1b). The natural course of the yolk sac is to increase in size until the tenth week of gestation and, thereafter, to involute, becoming undetectable by ultrasound by approximately 12–15 weeks. By 6 weeks, a small embryo with measurable crown rump length (CRL) and heart rate is visualized (Fig. 1c). This embryo will grow over the course of the first trimester, differentiating into cephalad and caudal poles around weeks 7–8, sprouting limb buds around weeks 8–9, and demonstrating movement around weeks 9–10.

Fig. 1
figure 1

Anatomy of the developing embryo. Expected appearance of the early pregnancy at 5 weeks (a), 5.5 weeks (b), and 6 weeks (c) on diagrammatic and gray-scale ultrasound images

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Complications of intrauterine pregnancy

Pregnancy failure

The ability to distinguish normal fetal development from early pregnancy failure is critical in the ED setting. A 2011 consensus conference recommended sonographic criteria for diagnosing non-viable pregnancy, with specificity as close to 100% as reasonably achievable [8]. These criteria are widely accepted by numerous national and international medical societies. Based on these criteria, sonographic findings are divided into ones that are diagnostic of, and others that are suspicious for, pregnancy failure (Figs. 2 and 3).

Fig. 2
figure 2

Definitive diagnosis of early pregnancy failure: criteria and representative images

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Fig. 3
figure 3

Findings suspicious, but not diagnostic of, pregnancy failure: criteria and representative images

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Subchorionic hematoma

Subchorionic hematoma refers to hemorrhage around a portion of the gestational sac, with blood collecting between the chorion and the uterine wall. This type of hemorrhage most frequently occurs during the first 20 weeks of gestation and can be associated with pelvic bleeding. On ultrasound, a subchorionic hematoma appears as a crescenteric heterogeneous collection subjacent to the chorion and can be echogenic or hypoechoic depending on its age. Ultrasound is commonly used to assess the presence and size of these collections. The size of a hematoma can be assessed relative to the gestational sac size and categorized as small, medium, or large [9]. Small- and medium-sized hematomas have a more favorable prognosis than large ones, with some studies suggesting that the rate of abortion doubles in large (18.8%) compared with small and moderate hematomas (7.7 and 9.2%, respectively) [10, 11]. Given this risk, patients with large first-trimester subchorionic hematomas should be followed closely.

Intrauterine device failure

The intrauterine device (IUD) has become a commonly used and highly effective method of pregnancy prevention, with some sources demonstrating them to be 98–99% effective [12]. However, pregnancy does rarely occur in women utilizing IUDs as their preferred method of contraception, with a failure rate reported to be approximately 1% (Fig. 4). In these cases, the pregnancy is at risk for a number of complications. Compared with women without IUD in situ, patients who elect to keep their IUD in place for the duration of the pregnancy are at increased risk for having vaginal bleeding, placental abruption, low-birth-weight fetuses, chorioamnionitis, and preterm delivery [13]. In addition, these patients have increased risk of spontaneous pregnancy loss compared with patients who conceived without IUD (16 vs 1%) [13, 14] and increased risk compared to patients who have their IUD removed early during the pregnancy (18 vs 14%) [13, 15].

Fig. 4
figure 4

Intrauterine pregnancy with IUD located within the uterus (arrow)

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The preferred management of patients with synchronous IUD and desired first-trimester intrauterine pregnancy is removal of the IUD under ultrasound guidance, especially if the location of the device suggests that it can be removed without high risk for disruption of the pregnancy. The removal of IUDs during the second trimester can be more challenging, with high risk if the device is imbedded within the placenta or gestational sac. During the third trimester, the risks of removal outweigh the benefits [16]. Thus, early diagnosis and assessment of IUD location are key for management of patients with synchronous IUD and intrauterine pregnancy.

Retained products of conception

When intrauterine fetal tissue persists following delivery, miscarriage, or termination of pregnancy, it is referred to as retained products of conception (RPOC). Pathologically, this is characterized by the presence of chorionic villi, which is fetal tissue that attaches to the decidua basalis of the endometrium and contains placental tissue and fetal capillaries.

RPOC may occur in as many as 3–5% of normal vaginal deliveries [17]. The clinical manifestations of RPOC are non-specific and can be similar to those following normal delivery, including post-partum hemorrhage, pain, or fever. RPOC occurs most frequently after second-trimester delivery or termination of pregnancy, but it has also been associated with failure to progress during delivery, placenta accreta, and instrument delivery [18, 19].

Diagnosis of RPOC relies on a constellation of clinical evaluation, laboratory results, and sonographic findings. Clinical symptoms are non-specific, and laboratory levels of white blood cell count and hCG are imperfect tools for diagnosis as they can be elevated following normal delivery. Gray-scale and color Doppler ultrasound can help make the diagnosis. Sonographic findings that are suspicious for RPOC include thickened endometrium (> 80% sensitive and 20% specific), presence of an endometrial mass (sensitivity 29%, PPV > 80%), and internal Doppler flow, with internal vascularity and thickened endometrium being associated with a PPV ~ 96% for RPOC [20,21,22]. RPOC can be excluded with a high degree of confidence in the absence of these sonographic findings [22].

Once RPOC has been diagnosed, management can be conservative, medical, or surgical. Medical management usually employs the use of prostaglandin E1 analogs, which facilitate the expulsion of products from the uterus, while surgical intervention can take the form of curettage or hysteroscopic removal.

Gestational trophoblastic disease

Gestational trophoblastic disease (GTD) represents a spectrum of disorders involving abnormal growth of placental trophoblastic tissue. The continuum of GTD ranges from benign hydatidiform moles to malignant invasive moles, choriocarcinoma, and the extremely rare epithelioid trophoblastic tumor (Fig. 5).

Fig. 5
figure 5

Complete molar pregnancy, demonstrating the uterine cavity expanded with echogenic material containing innumerable cystic spaces

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It is estimated that incomplete or partial hydatidiform moles occur in three per 1000 pregnancies and complete hydatidiform moles occur in one per 1000 pregnancies [23, 24]. Partial moles are triploid, containing two sets of chromosomes from the father and a single set from the mother. In contrast, complete moles are diploid, resulting from the fertilization of an empty egg with two sets of paternal chromosomes.

Diagnosis of molar pregnancies depends on a combination of elevated hCG, clinical symptoms, and sonographic findings. The most common presenting symptom is bleeding during early pregnancy, with previously reported features such as anemia, uterine enlargement, hyperemesis, and respiratory distress becoming more rare as diagnosis is made earlier in the first trimester rather than late-second trimester [24, 25]. On ultrasound, complete mole appears as a heterogeneously echogenic mass within the uterus, with multiple cystic spaces and no identifiable fetal tissue. Theca lutein ovarian cysts are also common. It is important to note that hydropic failed pregnancies can resemble complete moles and have been misdiagnosed in up to 10% of cases [26]. Partial moles are characterized sonographically by the presence of an identifiable but growth-restricted and often abnormally formed fetus, as well as a thickened placenta that may contain cysts. However, some partial moles can also have a sonographic appearance indistinguishable from complete mole or hydropic failed pregnancy [27]. Thus, in order to confirm histologic type, products of conception from non-viable pregnancies should undergo pathologic evaluation.

Malignant forms of GTD include invasive molar pregnancies, choriocarcinoma, and epithelioid trophoblastic tumor. Invasive molar pregnancies result when a complete or partial mole invades the myometrium. It is estimated that 16% of complete moles and 0.5% of partial moles undergo malignant transformation into invasive moles, choriocarcinoma, or epithelioid trophoblastic tumor [28,29,30]. Although locally aggressive, invasive moles maintain chorionic villus structure and rarely metastasize. This is in contrast to choriocarcinomas, which lose their characteristic chorionic villous architecture and tend to have early hematogeneous metastases (primarily to lung and vagina) even when the primary tumor is small [29, 30]. Choriocarcinomas are typically hemorrhagic and necrotic on gross examination and can arise from molar pregnancy as well as term pregnancy, miscarriage, or ectopic gestations. Epithelioid trophoblastic tumor is the rarest form of GTD arising most commonly from normal pregnancies, but it can also arise from molar and failed pregnancies. Its growth pattern is slower than choriocarcinomas, and it tends to metastasize locally, in contrast to the hematogeneous and distal metastatic disease seen in choriocarcinoma. Invasive mole, choriocarcinoma, and epithelioid trophoblastic tumor are sonographically indistinguishable from one another and typically present with echogenic highly vascular uterine masses. The presence of distant metastatic disease is better appreciated with use of chest radiography, computed tomography, or magnetic resonance imaging.

Stratification of disease severity and decision to treat GTD should be based on the Federation of Gynecology and Obstetrics prognostic scoring and anatomical staging system [31]. A score of 0–6 suggests low risk of resistance to monotherapy, while scores > 7 indicate higher risks for resistance to monotherapy and will likely require multidrug treatment. Indications for chemotherapy for GTD include rising hCG concentration after evacuation, intraperitoneal hemorrhage, pathologic diagnosis of choriocarcinoma, evidence of widespread metastatic disease, and elevated hCG concentration 6 months after evacuation. A rising hCG is defined as two consecutive increases in hCG concentration of 10% or more for at least 2 weeks (i.e. days 1, 7, and 14) [24, 32].

Ectopic gestation

Ectopic pregnancy (EP) occurs when a fertilized oocyte implants outside the body of the uterus. It is estimated that EPs affect 1–2% of normal gestations and 2–5% of pregnancies achieved from assisted reproductive technology [33, 34]. Although maternal mortality has declined from 3.5 in 10,000 pregnancies during the 1970s to 2.6 in 10,000 pregnancies in 1992, EP remains the most common cause of death during the first trimester of pregnancy [35, 36], with ruptured ectopics accounting for 6% of all maternal deaths [37].

The main risk factors for EP include pelvic inflammatory disease, prior history of ectopic gestations, and prior gynecologic surgery [35]. Other factors contributing to a mother’s risk of EP include history of infertility, congenital uterine anomalies, history of smoking, advanced maternal age, and endometriosis.

Clinical presentations of EP are variable. Classic symptoms include amenorrhea, pelvic pain, and vaginal bleeding, but fewer than 50% of patients with confirmed EP present with the classic clinical findings [34].

Initial evaluation of patients with suspected EP includes a thorough clinical history, quantitative hCG, and transvaginal ultrasound. Familiarity with the classic sonographic findings of both tubal and non-tubal ectopic pregnancies is key for early diagnosis and treatment with ectopic gestations. The following will outline both the ultrasound findings and potential treatment algorithms for patients with EPs.

Tubal

The fallopian tube is the most common site of EP, accounting for more than 90% of all ectopic gestations. Within the tube, the ampulla is the most common site of implantation (70%) [35, 38, 39], possibly because most fertilization occurs within this portion of the tube. The isthmus (12%) and fimbria (11%) are less common sites of tubal implantation. Definitive sonographic findings of a tubal ectopic pregnancy include an adnexal mass containing a yolk sac or embryo, with or without cardiac activity (Fig. 6). Other findings suspicious of a tubal EP include a tubal ring or an extraovarian adnexal mass. When there is uncertainty about whether an adnexal mass is intraovarian (representing a corpus luteum) or extraovarian (representing an EP), the distinction can be made by observing whether it moves together or separately from the ovary when pressure is applied via the transvaginal probe. The positive predictive value of a mobile adnexal mass in a symptomatic patient with a positive hCG and no intrauterine pregnancy is over 90% [8].

Fig. 6
figure 6

Definitive finding of tubal ectopic pregnancy. Sonographic image demonstrates a right adnexal mass with yolk sac (arrow) that is distinct from the right ovary (RO) and uterus

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The two most common treatment methods for EP are laparoscopic resection of the ectopic gestation and medical management with methotrexate (MTX), a dihydrofolate reductase inhibitor that interferes with DNA and RNA precursor synthesis. MTX has been shown to be more cost-effective than surgical intervention while maintaining similar rates of treatment efficacy and preservation of future fertility [40,41,42]. However, some contraindications to systemic MTX exist (Table 1), and these factors must be considered when navigating the treatment algorithms for tubal ectopic pregnancies.

Table 1 Management of tubal ectopic pregnancy
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Non-tubal

Implantation of an ectopic gestation outside of the fallopian tube accounts for less than 10% of all EPs. There are four main types of non-tubal EP: interstitial, cervical, ovarian, and abdominal. Implantation in a cesarean scar, while not technically an ectopic pregnancy, is another type of abnormal pregnancy location. These entities will be discussed in the sections to follow.

Interstitial ectopic pregnancy

Interstitial ectopic pregnancy accounts for 2–4% of all EPs and involves the implantation of the gestational sac in the portion of the fallopian tube traversing the muscular wall of the uterine cornu, at the junction with the Fallopian tube. The interstitial portion of the fallopian tube is somewhat distensible and allows for these gestations to advance further than the other EPs, sometimes as late as 16 weeks. As a result, interstitial ectopic pregnancies are associated with higher rates of hemorrhage and mortality, 2.5% compared to 0.4% of other EPs [43, 44].

Classic sonographic findings associated with interstitial ectopic pregnancy include eccentric location of the GS within the uterine cavity, with virtually no overlying myometrium, and a bulged contour of the superolateral aspect of the uterus. When the 2D gray-scale images are equivocal about whether a pregnancy is located normally or interstitially, 3D coronal reconstruction is often useful to confirm the location of the gestational sac.

Management of interstitial ectopic pregnancies can be medical or surgical, depending on factors including the hemodynamic stability of the patient and patient preference [46].

Cervical ectopic pregnancy

Cervical ectopic pregnancy occurs when the gestational sac implants within the cervix. This form of ectopic gestation is rare, accounting for less than 1% of all ectopic pregnancies. It occurs more frequently in pregnancies achieved via in vitro fertilization than in naturally conceived pregnancies. Other risk factors include history of prior uterine curettage [35, 47], Asherman syndrome, and presence of IUD [46,46,48].

Clinically, patients with cervical ectopic pregnancy usually present with vaginal bleeding, pelvic pain, and a positive pregnancy test. Sonographic evaluation is the mainstay of diagnosis. It is important to note that cervical ectopic pregnancy can closely resemble a miscarriage-in-progress, since the latter may be located within the cervix at the time of the sonogram. In order to avoid this potential diagnostic pitfall, radiologists must be familiar with the sonographic findings of both entities.

A cervical ectopic pregnancy appears on ultrasound as a non-mobile, well-formed gestational sac (with or without a heartbeat) in the cervix. A miscarriage-in-progress, on the other hand, generally appears as a flattened, often irregular, gestational sac in the cervix that contains no heartbeat and may move when pressure is applied with a transvaginal probe (Fig. 7). When the distinction between these two diagnoses based on ultrasound findings is uncertain and the patient is hemodynamically stable, a follow-up scan performed in 24 h is helpful. If the gestational sac is still seen within the cervix, the likely diagnosis is cervical ectopic pregnancy, while disappearance of the sac would favor a miscarriage in progress.

Fig. 7
figure 7

Miscarriage in progress. Gestational sac with yolk sac and embryo was seen in the miduterus early during the ultrasound. Later during the examination, the gestational sac and its contents have an irregular contour and have migrated caudally, as the gestation makes its way out of the uterine cavity into the cervix

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Management of cervical ectopic pregnancies can be medical or surgical, depending on the hemodynamic stability of the patient and patient preference.

Ovarian ectopic pregnancy

Ovarian ectopic pregnancy results from the implantation of a fertilized egg within the ovarian parenchyma. Ovarian ectopic pregnancies are thought to affect 1:7000 to 1:30,000 pregnancies or 0.2–0.7% of all ectopic pregnancies. Although rare, the apparent incidence of ovarian ectopic pregnancies has been increasing, due in part to improved awareness and better diagnostic techniques. Risk factors for ovarian ectopic pregnancies are similar to other non-tubal ectopic pregnancies and include history of prior ectopic pregnancies, in vitro fertilization, and prior pelvic infections. However, some studies have suggested that the presence of an intrauterine device may be an independent risk factor for ovarian ectopic pregnancy [46, 49, 50].

Transvaginal ultrasound is the mainstay for diagnosis of ovarian ectopic pregnancies. The major challenge of diagnosing ovarian ectopic pregnancies is that an intraovarian mass in a patient with a positive pregnancy test is highly likely to represent a corpus luteum, which can have a highly variable sonographic appearance. Thus, an intraovarian mass should be diagnosed as an ovarian ectopic pregnancy only if it contains a yolk sac or embryo with heartbeat (Fig. 8).

Fig. 8
figure 8

Ovarian ectopic pregnancy. A gestational sac is seen within the ovarian parenchyma

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Another challenge in diagnosing ovarian ectopic pregnancy is distinguishing it from a tubal ectopic adjacent to the ovary. To make this distinction when scanning in real time, pressure is applied with the transvaginal probe. If the gestational sac and ovary move as a single unit, the findings would be more consistent with an ovarian ectopic pregnancy. If they move together, given that ectopic implantation in the ovary is far more rare than in the fallopian tube, it is far more likely to be a tubal ectopic than an ovarian ectopic.

Management of ovarian ectopic pregnancies is usually surgical, with little data available regarding medical management with MTX [51,51,52,54].

Abdominal ectopic pregnancy

Abdominal ectopic pregnancy results from implantation of the gestational sac outside the uterus in the peritoneal cavity. The most common implantation sites include the anterior and posterior uterine pouches as well as the uterine and adnexal serosa. Implantation on omentum, bowel, liver, and spleen is less common [46, 55]. Abdominal ectopic pregnancies are rare, accounting for approximately 1.3% of all ectopic gestations and are classified as primary or secondary [38]. Primary abdominal ectopics result from fertilization of an egg within the peritoneal cavity. More common, secondary abdominal ectopics result from rupture of an undetected tubal or ovarian ectopic pregnancy into the peritoneal cavity. Pelvic inflammatory disease, endometriosis, and use of assisted reproductive technologies are all risk factors for abdominal ectopic pregnancy [46, 56].

Abdominal ectopic pregnancies are associated with a high rate of morbidity and mortality. The maternal mortality of abdominal ectopics has been shown to be 7.7 times higher than that of tubal ectopics and 90 times higher than that of intrauterine pregnancy [55, 57]. The high rate of morbidity is due in part to delay in diagnosis as well as maternal hemorrhage in the setting of placental separation and trophoblastic invasion of maternal abdominal organs.

Transvaginal and abdominal sonographies are used to diagnose abdominal ectopic gestations. Findings that suggest an abdominal ectopic on ultrasound include an empty uterus, intraperitoneal gestational sac likely surrounded by bowel, placenta outside the confines of the uterus, and absence of more common tubal or cesarean-scar implantations [48]. Once diagnosed, intervention for resolution is recommended. Rare reports of expectant management can be considered in patients diagnosed with an abdominal ectopic after 20 weeks, stable maternal condition, no signs of fetal growth malformation, and placental implantation outside of the upper abdomen. Delivery is recommended at 34 weeks, and the placenta is frequently left in situ given high risk for hemorrhage [46, 58, 59].

Cesarean-scar implantation

Cesarean-scar pregnancy implantation involves implantation of the gestational sac within the myometrium at the site of prior anterior lower uterine segment hysterotomy. The vascular supply to the lower uterine segment is poor, leading to incomplete wound healing and scarring of the endometrium following a cesarean-section, thus providing an opportunity for the gestational sac to implant on the myometrium [60]. These abnormal pregnancy implantations are quite rare, but as the rate of cesarean-sections has increased, there has been a concordant increase in the incidence of cesarean-scar implantations.

Because of the intramural location of the gestational sac, growth of the sac can lead to uterine rupture and substantial blood loss. Depending on the exact site of the implantation, the expanding gestational sac can grow anteriorly through the myometrial defect or posteriorly into the uterine cavity. The former can lead to uterine rupture and massive hemorrhage. The latter can go undiagnosed and proceed later into pregnancy, often leading to placenta accreta.

Transvaginal ultrasound is the primary imaging modality for diagnosing cesarean-scar implantation. Sagittal images of the uterus are ideal for locating the gestational sac at the anterior lower uterine segment hysterotomy site. The gestational sac usually has a triangular configuration (Fig. 9) and extends to, or close to, the anterior serosal surface of the uterus. In addition, the midportion of the uterus, as well as the cervix, should be visible and empty.

Fig. 9
figure 9

Cesarean-scar implantation. Gestational sac is seen within the myometrium of the anterior lower uterine segment, corresponding to the site of prior hysterotomy. The gestational sac has a triangular configuration and there is thinning of the myometrium overlying the sac

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Management of cervical ectopic pregnancies can be medical, surgical, or (in some cases) expectant [46, 61,61,63].

Assisted reproductive technology

Background

Assisted reproductive technology (ART) involves the use of fertility treatments that manipulate both egg and sperm outside the body in order to aid couples in achieving a successful pregnancy. It is estimated that 12% of women aged 15–44 have used infertility services and that the use of ART has doubled in the United States over the past decade [64]. Given the prevalence and increasing utilization of ART, familiarity with the procedures and the common complications is important for any emergency medicine professional who deals with pregnancy complications.

The most common form of ART is in vitro fertilization (IVF). IVF involves stimulation of the ovaries using gonadotropin injection, egg retrieval, fertilization in a laboratory, and transfer of the embryo into a uterine cavity. This is in contrast with ovulation induction, which involves less powerful follicular stimulation, most commonly with clomiphene citrate, and in vivo oocyte fertilization. Of note, ovulation induction is not strictly considered to be under the umbrella of ART as it does not involve manipulation of both the oocyte and sperm [65]. ART has been used successfully to treat a variety of causes of infertility, including tubal occlusion, polycystic ovarian syndrome, ovarian failure, and severe male factor infertility [66].

The common complications associated with ART during the first trimester involve sequelae of potent ovarian stimulation and implantation of multiple embryos, namely ovarian torsion and heterotopic pregnancy.

Common complications

Ovarian torsion

Ovarian torsion involves the twisting of the ovarian pedicle with resultant obstruction of the venous, lymphatic, and eventually arterial flow. Torsion is more common on the right than the left, with a ratio of 3:2. Some authors have suggested that the right-sided predominance is due to the fixed nature of the sigmoid colon, which limits the mobility of the left ovary and prevents against torsion [67, 68]. Risk factors for ovarian torsion include previous tubal ligation, polycystic ovarian syndrome, ovulation induction or stimulation, and presence of an ovarian mass.

It is estimated that 12–25% of torsion cases occur during pregnancy [68,68,70] with an incidence of 1–10 per 10,000 spontaneous pregnancies [71]. However, the incidence increases dramatically after ART to 6% and as high as 16% in those patients who suffer from ovarian hyperstimulation syndrome [70, 72, 73]. Most cases of torsion in pregnant women occur during the first trimester. However, torsion can occur at any point during pregnancy, and a high index of suspicion in a woman with symptoms suggestive of torsion is needed to ensure rapid diagnosis and treatment.

Most pregnant patients with torsion present with acute onset of pelvic or abdominal pain, usually localized to a single side. The pain may wax and wane as the ovary torses and detorses. In patients with these symptoms, transvaginal sonography is the primary diagnostic test. The most consistent sonographic finding for ovarian torsion is asymmetric ovarian enlargement [74] with ovaries greater than 4 cm being highly suspicious for torsion in the appropriate clinical setting [75]. Another frequent finding is abnormal location of the ovary, either midline or superior to the uterine fundus. The presence of an ovarian mass, particularly those greater than 5 cm should raise suspicion for potential torsion [67]. Additional sonographic findings of torsion include the presence of free fluid in the cul-de-sac, which can be seen in up to 87% of cases [74], and the presence of a twisted vascular pedicle, which has been shown to be 88% sensitive and 87% specific [76].

Doppler imaging is somewhat helpful in the evaluation of a woman with suspected torsion. Some studies suggest that the absence of venous flow is highly predictive of torsion with positive predictive values of 94% [77, 78]. Asymmetric vascular flow to the ovaries should raise suspicion for ovarian torsion, with diminished venous flow being more common than abnormal arterial flow [74]. However, it is important to note that Doppler can be falsely normal in cases of torsion, particularly in pregnant patients compared to their non-pregnant counterparts, with one study demonstrating false negative rates of 61% in pregnant patients compared to 45% in non-pregnant patients [70].

Heterotopic pregnancy

Heterotopic pregnancy refers to the simultaneous implantation of at least two embryos, one in normal location within the uterus and a second in an ectopic location. The reported rates of heterotopic pregnancy are variable but are estimated to occur in 1:4000–1:30,000 pregnancies. It is much more common in ART pregnancies, with reported rates as high as 1:100 [34, 46, 79]. Thus, in patients undergoing IVF and presenting with a positive HCG and intrauterine pregnancy, the EM physician should consider the possibility of concomitant ectopic gestation.

Patients with heterotopic pregnancy usually present with abdominal pain, peritoneal irritation, and enlarged uterus, but these findings are non-specific and can be seen in normal pregnancy or with a single ectopic gestation. As a result, heterotopic pregnancies often are diagnosed later than single ectopic pregnancies, and as many as 50% of patient presentations can be in the setting of ectopic rupture [80].

Standard serum hCG levels are not reliable in the case of multiple gestations. Ultrasound is the major diagnostic tool for evaluation of patients with suspected or high risk of heterotopic pregnancy. Sonographic findings include both an intrauterine gestation as well as an additional gestation either in the adnexa (88%), abdomen (2.7%), interstitium (6.3%), ovary (0.9%), or cervix (1.8%) [80] (Fig. 10).

Fig. 10
figure 10

Heterotopic pregnancy. There is an intrauterine pregnancy (IUP) as well as an adnexal gestational sac containing a yolk sac (arrow)

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Treatment of heterotopic pregnancy is variable, depending on the stability of the patient and the location of the EP.

Conclusion

Embryologic development is a stepwise process, and knowledge of the normal and abnormal sonographic findings in early pregnancy is necessary for prompt and accurate diagnosis and management of women with abnormal gestations. First-trimester emergencies come in a wide variety of types, some of which present with vague symptoms and have subtle imaging findings. Familiarity with treatment algorithms for first-trimester emergencies is crucial for effective communication between the radiologist and the treatment team. While careful assessment for pregnancy complications should be undertaken in any woman undergoing early first-trimester imaging, it is critical to have a high index of suspicion in women who present with pain or bleeding and in those whose pregnancies were achieved via assisted reproductive technology.