Introduction

Preeclampsia is one of the most common causes of maternal and fetal morbidity and mortality [2]. Its incidence is 4–8% of pregnancies [24, 43].The pathophysiological mechanism is characterized by a failure of the trophoblastic invasion of the spiral arteries which may be associated with an increased vascular resistance of the uterine artery and a decreased perfusion of the placenta [28]. But, the exact etiology of preeclampsia is still unknown. Various hypotheses in the occurrence of preeclampsia have been set including the increase of thromboxane levels and the decrease of prostaglandin levels as well as the imbalance of lipid peroxidase and antioxidants. However, the results from many clinical studies show the relationship between the aggravation of the hypertension complication and the change in concentrations of various chemicals in mother’s serum [3, 6–8, 13, 22, 25, 26, 35, 42]. Interestingly, the significant reduction in serum calcium and magnesium are found in preeclamptic mothers [12, 30, 38, 45]. This result agrees with the physiological roles of both calcium and magnesium in humans.

On the physiological basis, calcium plays an important role in muscle contraction and regulation of water balance in cells. Modification of plasma calcium concentration leads to the alteration of blood pressure. The lowering of serum calcium and the increase of cellular calcium can cause an elevation of blood pressure in preeclamptic mothers. Besides the serum calcium, the serum magnesium also decreases in women with preeclampsia. Generally, magnesium has been known as an essential cofactor for many enzyme systems. It also plays an important role in neurochemical transmission and peripheral vasodilation. Moreover, magnesium sulfate has been used as the drug of choice in severe preeclampsia and eclampsia treatment [7]. Therefore, the modification of calcium and magnesium metabolism during pregnancy could be one of the potential causes of preeclampsia [16, 21, 38]. However, the role and status of serum calcium and serum magnesium in pregnant women are still being discussed. The aims of this study are to measure serum levels of calcium and magnesium in preeclamptic pregnancy and to compare with those in normal pregnancy.

Materials and methods

The Ethics Committee of the Faculty of Medicine, Chulalongkorn University has approved the study protocol, and informed consent was obtained from each participant before their recruitment. The study population consists of 40 pregnant women with diagnosis of preeclampsia, and 40 normal pregnant women, who were admitted to the labor room of the Department of Obstetrics and Gynecology, Faculty of Medicine, Chulalongkorn University from June 2003 to January 2004. All pregnant participants were in their third trimester. Preeclampsia is defined as a blood pressure of at least 140/90 mmHg measured on two occasions each 6 h apart, accompanied by proteinuria of at least 300 mg per 24 h, or at least 1+ on dipstick testing [2]. Severe preeclampsia is defined as having one or more of the following criteria: blood pressure of at least 160/110 mmHg measured on two occasions each 6 h apart, proteinuria of at least 5 g per 24 h, or at least 3+ on dipstick testing, oliguria of less than 500 ml per 24 h, cerebral or visual disturbances, pulmonary edema or cyanosis, epigastric or right upper quadrant pain, impaired liver function, thrombocytopenia, fetal growth restriction [2]. Fetal growth restriction is defined as the condition in which the newborn has birth weight less than 10% for gestational age [1]. Patients with history of chronic or transient hypertension, history of renal disease or cardiovascular disease, diabetes, multiple gestations were all excluded.

On admission to the labor room, venous serum samples were anaerobically collected when the patients were in the supine position, prior to their commencement to intravenous therapy. The venous blood was aspirated from the participants’ antecubital vein, placed in a plain vacuumtainer tube. At the time of blood collection, urine protein, edema and deep tendon reflexes were assessed. Urine protein was measured by dipstick from a random urine sample and was graded on a scale of 0 to 4+ (0, none; 1+, 30 mg/dl; 2, 100 mg/dl; 3+, 300–1,999 mg/dl; 4+, at least 2,000 mg/dl). Pedal and pretibial edema were assessed by palpation and related on a scale of 0 to 4+ (0, none; 1+, generalized puffiness; 2+, indentation depth up to 1 cm with immediate recovery; 3+, indentation depth 1–1.5 cm with slow recovery; 4+, indentation depth greater than 1.5 cm). Deep tendon reflexes were assessed by striking the patellar tendon and grading the response on a scale of 1+ to 4+ (1+, slightly movement of the foot; 2+, mild leg withdrawal; 3+, marked leg and foot movement; 4+, abrupt leg jerk and extension). The blood sample obtained on admission was also sent for complete blood count, blood urea nitrogen, creatinine, uric acid, serum albumin, serum alkaline phosphatase, prothrombin time and partial thromboplastin time.

Both preeclamptic and normal pregnant women were interviewed to access their daily diet and medicine, such as vitamin and calcium supplement, milk, fish, cereal consumed or used in 1 week prior to the study.

Blood samples were allowed to clot at room temperature and then centrifuged at 3,000 rpm for 10 min. Serum aliquots were stored at −20°C until analysis. Serum calcium and magnesium were measured colorimetricaly with Hitachi spectrophotometer Model 912. The coefficient of variance within run and between run of serum calcium were 0.9% and 1.5%, respectively. The coefficient of variance within run and between run of serum magnesium were 1.2 and 1.4%, respectively.

The data were analyzed with the SPSS software package version 10.0 for Windows (SPSS, Chicago, IL, USA) and expressed in terms of mean, standard deviation and percentage. Continuous variables were compared with the Student’s t-test while the χ 2-test (or Fisher exact tests when appropriate) were used to compare the frequencies of pregnancy outcomes. A p value <0.05 was considered to be statistically significant.

Results

Clinical characteristics according to participants are shown in Table 1. Age and parity between normal and preeclamptic pregnant women are not significantly different. Gestational age and neonatal birth weight are less in the preeclamptic group, while total weight gain, body mass index (BMI), systolic blood pressure and diastolic blood pressure are higher in the preeclamptic group with statistical significance. Cesarean sections were performed in the preeclamptic group more than in the normal group (67.5 vs. 25%, p<0.05). Five cases of intrauterine growth retardation (IUGR) were found in the preeclamptic group, whereas no case of IUGR was found in the normal group.

Table 1 Clinical characteristics according to participants. BMI body mass index, IUGR intrauterine growth restriction
Full size table

The questionnaires to recall about dietary intake in 1 week before enrollment into the study showed that the dietary intake in preeclamptic and normal pregnant women was of no significant difference. Most of the preeclamptic and normal pregnant women always took iron supplement and drank milk, ate fish and green vegetable daily. Butter, sunflower seed, pumpkin seed, yogurt, nut almond, cod liver oil and cereal did not seemed to be preferred sources of calcium of our participants. They did not eat these items often.

The serum calcium concentration in preeclamptic pregnant women was significantly lower than that of normal pregnant women (9.0±0.4 mg/dl vs. 9.7±0.7 mg/dl, p<0.0001). Like serum calcium, serum magnesium concentration in preeclamptic women was significantly lower than that of normal pregnant women (0.77±0.08 mmol/l vs. 0.85±0.09 mmol/l, p=0.001; Table 2).

Table 2 Serum levels of calcium and magnesium according to participants
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Discussion

We found a decrease in both serum calcium and magnesium in preeclamptic pregnant women as compared to normal pregnant women. These findings confirmed the hypothesis that hypocalcemia and hypomagnesemia may be the etiologies of preeclampsia [16, 21, 38].

The mean serum calcium levels in these normal pregnant women (9.7±0.7 mg/dl) were within the range (9.5–11.1 mg/dl) given by previous reports [21, 45]. The serum calcium level did not change throughout pregnancy in some studies [23, 38, 44], but it significantly decreased in the second trimester in one study [32]. The mean serum calcium level in preeclampsia was less than in normal pregnancy from our study. The data support the hypothesis that calcium might be a cause in the development of preeclampsia. The effect of serum calcium on changes in blood pressure could be explained by the level of cellular concentration of calcium. The increase of cellular calcium concentration when serum calcium went lower led to constriction of smooth muscles in blood vessels and increase of vascular resistance [29]. This finding is similar to the findings of previous studies [14, 16, 21, 39, 45]. However, this is contradictory to some studies that the mean serum calcium level in preeclampsia was not different from normal pregnancy [27, 31]. The negative results of the other studies may be caused by the mislead between underlying chronic hypertension or renal disease and preeclamptic condition during pregnancy, the difference of dietary intake as well as the difference in the time of sample collection.

The mean serum magnesium levels in these normal pregnant women (0.85±0.09 mmol/l) was within the range (0.65–1.11 mmol/l) given by previous reports [11, 34, 45]. Like serum calcium, the mean serum magnesium level in preeclampsia was lower than in normal pregnancy from our study. Our data support the hypothesis that magnesium deficiency is the cause of preeclampsia. The serum magnesium level decreased significantly during pregnancy [36, 38]. Generally, the hypomagnesemia in most pregnant women is associated with hemodilution, renal clearance during pregnancy and consumption of minerals by the growing fetus [15, 38]. Recent studies [38] report a relationship between hypomagnesemia and pregnancy induced hypertension. Although the explanation for this result is not clear, they propose that magnesium promotes vascular muscle relaxation [7, 29]. But, this result is contradictory to some studies which report that the mean serum magnesium level in preeclampsia is not different from that of normal pregnancy [9, 11, 19, 39, 45]. Some studies show that serum magnesium is even higher in the preeclamptic group than in normal pregnancy [34, 36]. The difference may be explained by the variation of the study population and the dietary intake.

The limitation of this study is the unknown amount of daily calcium and magnesium intake of each participant. However, we collected data of the daily intake of food and drugs within 1 week before the enrollment of the participants for the study. We found that amount of calcium and magnesium intakes are not different between the preeclamptic women and that of normal pregnant women. Thus, the lower serum calcium and magnesium levels in the preeclamptic group than normal pregnancy are not contributed by the difference of their intake.

According to the results from our research, it is likely that the intake of calcium and magnesium supplement may help to reduce the incidence of preeclampsia in our population. This idea/assumption agrees to a recent study performed by Kulier et al. [17]. The prescription of calcium and magnesium supplement in pregnancy could reduce the incidence of preeclamptic complication during pregnancy [4, 5, 17, 33]. The recommended dietary allowance (RDA) in USA recommends that pregnant women should take 1,000–1,500 mg of calcium daily for preeclamptic complication prevention [41, 10]. Regarding the role of magnesium supplement, there is no strong evidence that clearly shows the therapeutic effect of magnesium in preeclampsia prophylaxis. However, due to the properties of vasodilatation and uterine relaxation of magnesium, a magnesium-rich diet as well as magnesium supplement should be an advantage to preeclamptic pregnant women in prevention of the aggravation of hypertension [40]. However, some studies show insignificant effects of calcium and magnesium supplement on preeclamptic complication [18, 20, 37]. The actual role of magnesium supplement and calcium supplement in preeclamptic prevention must be further investigated.

Our study shows that both serum calcium and serum magnesium in preeclamptic pregnant women were lower than in normal pregnant women. These findings support the hypothesis that hypocalcemia and hypomagnesemia are possible etiologies of preeclampsia. However, calcium and magnesium supplement in pregnant women for the prevention of preeclampsia requires further study.