Underweight women are more likely than those of normal weight to have a preterm delivery or a low-birth-weight infant, but their risk of most complications of pregnancy and other poor outcomes is no greater than that of normal-weight women, according to findings from a population-based British study.1 Moreover, the prevalence of a number of adverse events--notably, preeclampsia, surgical obstetric interventions and postpartum hemorrhage, all of which contribute to the risk of maternal mortality--is lower among underweight women than among normal-weight women.

Using a database containing information from virtually all maternity units in the North West Thames section of London, the researchers analyzed the records of women who had a singleton pregnancy between 1988 and 1997. They calculated women's body mass index (weight divided by the square of height) at the start of prenatal care, and compared the experiences of roughly 38,000 women who were underweight (i.e., with a body mass index of less than 20) with those of 177,000 normal-weight women (i.e., who had an index of more than 20 but less than 25).

Most of the women were white (64% of those who were underweight and 73% of their normal-weight counterparts); about half of each group had not given birth before, and one-third had had one previous birth. When the women first visited their maternity care facility, 3-4% had hypertension; negligible proportions had diabetes.

The researchers examined four types of variables: prenatal, maternal and delivery complications, and pregnancy outcomes. They assessed the independent effect of body mass index through a series of multiple logistic regression analyses that controlled for maternal age, ethnicity, parity, history of hypertension and diabetes, and additional factors of specific importance for individual variables.

Body mass index was significantly associated with four prenatal complications. Underweight women were more likely than normal-weight women to be anemic (odds ratio, 1.3). However, they were less likely to develop preeclampsia (0.8) or gestational diabetes (0.6), or to begin care late in pregnancy (0.4). The odds of placenta previa, placental abruption and breech presentation were similar in both groups of women.

Maternal complications (e.g., genital or urinary tract infection, pulmonary embolism and wound infection) were rare, affecting no more than 1% of women in either group. Roughly 20% of both underweight and normal-weight women required a prolonged postpartum hospital stay. The likelihood of these events did not differ significantly according to women's body mass index.

All seven delivery-related complications that the investigators examined were associated with body mass index. Underweight women had significantly elevated odds of having a breech delivery (1.3) but reduced odds of requiring induced labor, an operative vaginal delivery, or an emergency or elective cesarean delivery (0.7-0.9). They also were less likely than normal-weight women to have a postpartum hemorrhage or major postpartum hemorrhage (0.8-0.9).

Two adverse pregnancy outcomes were more common among underweight women than among their normal-weight counterparts: Women with a low body mass index had an increased risk of bearing an infant whose weight was below the fifth percentile (1.4). They also had elevated odds of delivering before 37 weeks' gestation (1.2), but not before 32 weeks, which the researchers note is a more "clinically important" threshold. Other outcomes that were significantly related to low body mass index were breastfeeding (0.9), having an infant with a birth weight above the 90th percentile (0.5) and admission to a special care baby unit more than 24 hours after giving birth (1.1). Delivery after 42 weeks, stillbirth and infants' Apgar scores were not affected by body mass index.

According to the investigators, the "main adverse outcomes" associated with a woman's being underweight early in pregnancy are an increased risk of having a low-birth-weight infant or a premature delivery, but the "clinical and pathological significance" of these associations is unclear. The researchers conclude that being underweight does not increase the risk of most problems that may occur during or just after pregnancy, and it may protect against some "major" factors related to maternal mortality.--D. Hollander


1. Sebire NJ et al., Is maternal underweight really a risk factor for adverse pregnancy outcome? a population-based study in London, British Journal of Obstetrics and Gynaecology, 2001, 108(1):61-66.