Infants born at term to mothers with pre-eclampsia have similar birth weights, on average, to those of infants born to women who do not experience this condition; however, infants born preterm to mothers with preeclampsia weigh significantly less than those born to women with normal blood pressure during pregnancy, according to a retrospective cohort study of births in Canada.1 Similar results were found in analyses comparing women who experienced gestational hypertension with normotensive mothers. Sixty-one percent of women with preeclampsia gave birth at term; thus, most infants born to women with the disorder had normal birth weights for their gestational age.
The study was based on the medical records of 97,270 women who delivered at 35 hospitals in Alberta between July 1991 and December 1996. Women who had used antihypertensive drugs before pregnancy and those who had multiple births, preexisting chronic hypertension, diabetes, gestational diabetes, cardiovascular disease or chronic renal disease were excluded, because these characteristics are associated with both preeclampsia or gestational hypertension and birth weight. The final sample consisted of 87,798 births.
Women were classified as having gestational hypertension or preeclampsia by their blood pressure and urine protein level. Gestational age was based on the date of the mother's last menstrual period, and was verified by first-trimester or early second-trimester ultrasound. Analyses of variance compared the mean birth weights of infants born to mothers with gestational hypertension or preeclampsia with the mean birth weights of infants born at the same gestation to normotensive women. Linear regression analyses were used to control for the confounding effects of maternal smoking, maternal age, parity, obesity, prior spontaneous and induced abortion, prior births of infants who were small or large for gestational age, anemia and premature rupture of membranes.
Eight percent of women in the overall population were aged 19 or younger, 54% were 20-30 and 37% were older than 30. Sixty percent had had previous births, and 27% had smoked during pregnancy. The incidence of hypertensive disorders was low: Four percent of women experienced gestational hypertension, and 2% experienced preeclampsia. Nine percent of births occurred preterm (i.e., at less than 37 weeks' gestation), and 7% of newborns were low-birth-weight (i.e., less than 2,500 g). Sixty-one percent of women with preeclampsia gave birth at term.
In univariate analyses, infants born preterm to women with preeclampsia weighed, on average, 179-464 g less than those born preterm to women with normal blood pressure during pregnancy; after adjustment for all potentially confounding variables, preterm infants of preeclamptic mothers weighed between 224 g and 665 g less than others, and the differences were statistically significant. Infants born before 37 weeks' gestation to women experiencing gestational hypertension weighed, on average, 190-434 g less than those born before 37 weeks to normotensive women; in multivariate analyses, the average difference ranged from 55 g to 485 g. There were no significant differences between the birth weights of infants born at term to women experiencing gestational hypertension or preeclampsia and the birth weights of infants of women with normal blood pressure. In analyses taking into account women's parity, no significant differences were found in the effects of preeclampsia and gestational hypertension on birth weight.
The researchers note that the finding that most babies born to preeclamptic women at term have normal fetal growth cannot be reconciled with the currently held belief that preeclampsia is caused by reduced blood flow through the placenta to the fetus. Instead, they suggest that preeclampsia is likely the result of multiple factors, and that there may be at least two types of preeclampsia: one that restricts fetal growth and often leads to preterm delivery and one that does not produce these effects. The researchers recommend that further research be done to determine "whether there is a difference between these two possible subtypes of preeclampsia in terms of onset and severity."--J. Rosenberg
1. Xu Xiong et al., Impact of preeclampsia and gestational hypertension on birth weight by gestational age, American Journal of Epidemiology, 2002, 155(3):203-209.