Hypertensive disorders of pregnancy remain the second most common cause of maternal mortality in the United States, accounting for 15% of all maternal deaths.¹ Preeclampsia is a major cause of maternal and perinatal morbidity and mortality worldwide, particularly in developing countries.²

Most cases of preeclampsia occur in nulli-parous women.^3,4 This disease is a multisystem disorder of unknown etiology, and placental ischemia is considered to have a major role in the pathogenesis of these complications.^5,6

Preeclampsia is associated with reduced intravascular production of prostacyclin and excessive production of thromboxane A2.^7,8

During the past two decades, numerous clinical trials were conducted to evaluate the effectiveness of various methods to prevent or reduce the incidence of preeclampsia.²

The results of several clinical trials and meta-analyses have suggested that calcium supplementation^9,10 or low-dose aspirin^11,12 reduces the incidence of preeclampsia. Other trials also have shown the beneficial effects of these compounds in reducing the occurrence of preeclampsia.^13,14

Our study was conducted to evaluate the effect of low-dose aspirin or calcium supplements, taken during pregnancy, on the incidence of preeclampsia in nulliparous healthy women in Isfahan, Iran.

The inclusion criteria were; nulliparity, single gestation, first prenatal visit before 20 weeks of gestation, systolic/diastolic blood pressure (BP) lower than 130/80 mmHg, and no proteinuria detectable by a dipstick. Subjects were excluded if they had a history of cardiovascular, renal or endocrinologic problems, medical or obstetric complications and those with known hazardous condition (multifetal gestation, hydatidi-form mole).

All cases received prenatal care according to the approved model.¹⁵ BP, bodyweight and maternal height were measured at an initial prenatal visit; at each subsequent prenatal visit, BP, body weight and urine protein measurements were taken. The women were examined by trained staff every 4 weeks through the 28 weeks of gestation, and every 2 weeks through the 36th week and weekly thereafter.

BP was measured with the subject in the sitting position by a certified examiner using a standard mercury sphygmomanometer (Riester, Germany). According to a published protocol,¹⁶ systolic BP was recorded with the appearance of the first Korotkoff sound, and diastolic BP with disappearance of the fifth Korotkoff sound. Maternal height and weight were measured according to routine, standard methods. Patients were considered to have mild preeclampsia if they demonstrated an increase of 30 mmHg in systolic or 15 mmHg in diastolic BP above the standard pressure. In addition, they should have demonstrated equal or greater than 300 mg/24 hours in urine collection, or in two random urine specimens obtained 4 hours apart and containing at least 1+ protein by the dipstick method. Severe preeclampsia was defined as BP equal or greater than 160/110 mmHg and 4+ protein by dipstick on two occasions 4 hours apart, according to the American College of Obstetricians and Gynecologists’ Bulletin.¹⁷ After rupture of the membranes, the urine specimens were collected by catheter.

Newborns were weighed with a beam balance-scale, within half an hour after birth. All the staff were carefully instructed at the beginning of the study on the methodology and several physicians supervised their performance. Ethical permission was obtained from the Deputy of Health Affairs of Isfahan University of Medical Sciences (IUMS).

Data were collected using a standardized questionnaire and included demographic, medical and obstetrical histories. Prenatal care records and hospital charts were also recorded. The newborn chart included the neonatal weight at birth, the presence of respiratory distress syndrome, sepsis, jaundice and intrauterine growth retardation (IUGR), according to Divon et al.¹⁸ Fetal or neonatal death was also included if required.

Data analysis was performed with SPSS 9 (SPSS Inc, Chicago, Il, USA) statistical software. Chi-square analysis was used for the incidence of preeclampsia, IUGR, preterm delivery, fetal and newborn morbidity, mortality, education and employment status. Analysis of variance (ANOVA) was used for age, height, weight, body mass index (BMI) and duration of gestation. For maternal weight gain, Duncan test was used following ANOVA. Descriptive parameters (mean ± SE, frequency and percentage for categoric data) and confidence interval (95%) were used for data presentation.

The mean ± SE of age in aspirin, calcium and control groups was 21.5 ± 0.21, 21.9 ± 0.28 and 21.2 ± 0.19 years, respectively. Three groups of women were similar in age, body weight, BMI, and degree of education (Table 1).

Table 1. Comparison of demographic characteristics, employment and education status in the three study groups.

* NS = not significant (p > 0.05); **Duncan test showed significant differences between the aspirin and control groups, and between the calcium and control groups.

Of the 990 healthy nulliparous women observed, 61 (6.2 %) developed preeclampsia. The frequency of preeclampsia occurrence in the group 1 (aspirin), group 2 (calcium) and control group 3 was 4.6%, 4% and 10.1%, respectively. There was a statistically significant difference between the aspirin and control groups (p = 0.007), and between the calcium and control groups (p = 0.005). In contrast, no significant difference between the aspirin and calcium groups was detected (p = 0.7) (Table 2).

The mean weight gain (± SE) during pregnancy in the aspirin, calcium and control groups was 10.7 ± 0.19, 10.6 ± 0.18 and 9.9 ± 0.19 kg, respectively. ANOVA followed by Duncan test showed a significant difference in weight gain during pregnancy between the aspirin and control (p < 0.05) groups, and between the calcium and control groups (p < 0.05).

The mean duration of pregnancy (± SE) in the aspirin, calcium and control groups was 39 ± 0.07, 38.6 ± 0.09 and 39 ± 0.07 weeks, respectively (p < 0.001). The rate of preterm delivery, defined as birth before 37 weeks of gestation, in the aspirin, calcium and control groups was 11.8%, 13.7 % and 8.9 % (not significant, p = 0.16 ).

The mean weight at birth (± SE) was 3,120 ± 24 g in cases treated with aspirin, 3,035 ± 23 g in those under calcium-D therapy, and 2,910 ± 22 g in the control group. ANOVA followed by Dun-can test showed that these values were statistically different in the aspirin and control (p = 0.001), aspirin and calcium (p = 0.001), and calcium and control groups (p = 0.003). The incidence of IUGR in the aspirin, calcium and control groups was 7.1 %, 10.7 % and 11.9 %, respectively. Finally, fetal and newborn morbidity and mortality were similar in three groups (Table 2).

Table 2. Pregnancy outcomes: frequency of preeclampsia, birth weight, intrauterine growth retardation (IUGR), preterm delivery and newborn morbidity.

Preeclampsia complicates 6% to 8% of all pregnancies with the majority of cases (75%) occurring during a mother’s first pregnancy.^1,3 Preeclampsia is a multisystem disorder of unknown etiology. During the past several years, numerous clinical trials described the use of various methods to prevent or reduce the incidence of preeclampsia.² The results of the present study, which was performed on 990 healthy nulliparous women in Isfahan, Iran, showed that the use of low-dose aspirin or calcium-D daily reduced the frequency of preeclampsia. There were statistically significant differences in preeclampsia between the aspirin and control groups (p = 0.007) and between the calcium and control groups (p =0.005).These results are supported by several randomized trials that suggested the effectiveness of low-dose aspirin^10,12 or calcium supplementation^13,19 in reducing the incidence of preeclampsia; however, findings of other clinical trials are not in agreement with our results.^3,14,20

The evaluation of literature focused on studies related to the causes of preeclampsia, especially marked vasoconstriction and its pathogenesis. Some hypotheses consider a primary role involving endothelial cell dysfunction, aberrations in prostaglandin synthesis, fatty acid and antioxidant metabolism and the renin-angiotensin system. Trials with aspirin or calcium supplementation based on these theories were appraised.²¹ Some reports suggest a role for nitric oxide in the hemodynamic adaptation during normal pregnancy and decreased production or action of nitric oxide in preeclampsia. These reports suggest that calcium supplementation prevents preeclampsia by increasing the production of vascular nitric oxide.^21,22

There is strong evidence to suggest the use of calcium supplementation and prophylatic low-dose aspirin in preventing or delaying the onset of preeclampsia. However, this evidence is only available in small randomized, placebo-controlled trials; the results of large, multicenter trials are generally disappointing. The controversies may in part be explained by lack of strict inclusion criteria, late starting of treatment, use of ill-defined endpoints and different timing of aspirin ingestion.^23,24 In the current study, no significant difference was detected in the rate of IUGR between the three groups. The mean fetal birth weight was significantly higher in the aspirin group than among subjects in the calcium and control groups (Table 2).

Fetal growth may be provoked by using low-dose aspirin, which reduces thromboxane A2 synthesis and therefore increases the ratio of prostacyclin to thromboxane, leading to improvement in uteroplacental circulation.⁸ Perinatal and newborn morbidity and mortality rates were similar among the three groups. We believe that the similarities might be due to improvements in antepartum surveillance, assessments of fetal well-being and newborn care.

We suggest that low-dose aspirin or calcium- D can be used as an effective and inexpensive preventive measure to reduce the risk of preeclampsia in healthy nulliparous pregnant women. This supplementation also had a desirable effect on newborn birth weight.

We recommend conducting further studies on the correction of prostacyclin deficiency. Prostacyclin plays a major role in abnormal vasoconstrictor-vasodilator ratio in preeclampsia.