Certain adverse changes in a woman's employment during pregnancy are related to reduced infant birth weight, according to a study conducted among a nationally representative sample of U.S. women.¹ Compared with women who remained employed at a financially adequate level during their pregnancy, those who became unemployed, involuntarily switched to part-time work or transitioned to work paying poverty wages bore lighter infants, had elevated odds of giving birth to a low-birth-weight infant or both. These associations appeared to be mediated in part by infant gestational age.

Using data from the National Longitudinal Survey of Youth, researchers studied 1,165 women who had a singleton first birth between 1981 and 1994, were interviewed both just before and during that pregnancy, and had been adequately employed (neither receiving poverty wages nor working part-time involuntarily) at the first interview. The women's social, demographic and health characteristics were ascertained during the interviews, and their infants' weight was recorded at birth. The researchers assessed associations between adverse changes in employment during pregnancy and infant birth weight, testing the influence of three factors potentially mediating these associations—prenatal care, infant gestational age and maternal weight gain during pregnancy.

At the prepregnancy interview, the women were about 25 years old and had had 13 years of education, on average. One-third were black or Hispanic. Fifty-eight percent were married, and 86% were living above the poverty level. The women averaged about 39 hours of work per week; 40% were employed as clerical workers, 30% as professional workers and 19% as manual laborers; 11% did not report their occupation. Most (87%) said they liked their job "very much" or "fairly well."

During pregnancy, the majority of women did not smoke (78%) or drink alcohol (61%), and 85% began receiving prenatal care in the first trimester; their average weight gain was about 15 kg. Although 73% of the women remained adequately employed while pregnant, 4% became unemployed and 7% became inadequately employedâ€"either switching to work paying poverty wages (6%) or switching to part-time work involuntarily (1%). The remaining 16% of the women left the workforce.

The mean gestational age of the infants was nearly 39 weeks. At birth, infants weighed 3,325 g, on average; 7% had a low birth weight (less than 2,500 g).

In bivariate analyses, compared with women whose work status did not change during pregnancy, women who became unemployed and women who involuntarily switched to part-time work gave birth to infants who were significantly lighter (by 220 g and 434 g, respectively). In addition, women who became inadequately employed had increased odds of bearing a low-birth-weight infant (odds ratio, 2.1). This association was due mainly to sharply elevated odds among women who involuntarily switched to part-time work (6.4). The prevalence of early prenatal care was lower among women who became unemployed or inadequately employed during pregnancy than among those who remained adequately employed, and women who involuntarily transitioned to part-time employment gave birth at a significantly shorter gestation than their counterparts whose employment status was stable.

A first multivariate analysis assessed associations between employment changes and infant birth weight as a continuous variable, taking into account other factors possibly influencing birth weight (maternal age, race and prepregnancy weight; alcohol use and smoking during pregnancy; and infant sex). In this analysis, compared with women who remained adequately employed during pregnancy, women who became unemployed or involuntarily switched to part-time work gave birth to infants who were significantly lighter, weighing 185 g and 418 g less, respectively.

When the trimester of initiation of prenatal care was added to the analysis, the preceding associations were essentially unchanged. When infant gestational age was further added, the association of birth weight with unemployment persisted, whereas that with involuntary part-time work did not; in addition, compared with women whose employment status did not change, those who switched to work paying poverty wages gave birth to infants who were significantly lighter (109 g less), as did those who left the labor force (85 g less). Finally, when maternal weight gain during pregnancy was added to the analysis, only two employment changes remained significantly associated with birth weight: Compared with women who remained adequately employed, those who became unemployed gave birth to infants weighing 155 g less, and those departing the workforce gave birth to infants weighing 98 g less.

A second multivariate analysis assessed associations between maternal employment changes and low birth weight in infants, taking into account two other factors possibly influencing birth weight (maternal race and prepregnancy weight). In this analysis, compared with their counterparts who remained adequately employed during pregnancy, women who became part-time workers without wanting to do so had sharply elevated odds of giving birth to an infant with low birth weight (odds ratio, 7.4).

When infant gestational age was added to the analysis, the preceding association was no longer significant, but odds were significantly elevated among women who transitioned to work paying poverty wages (odds ratio, 3.3). When maternal weight gain during pregnancy was added to the analysis, switching to poorly paying work remained significantly associated with an increased likelihood of having an infant with low birth weight (3.2).

Commenting on the findings, the investigators speculate that stressors associated with financially inadequate employment, such as decreased social interaction, reduced income and psychological distress, may have adverse physiologic effects during pregnancy that lead to reduced intrauterine growth or preterm birth; these conditions, in turn, may explain the observed associations. The study, they caution, focused on a relatively advantaged group of women, who initially were adequately employed and experienced a change in employment; therefore, the findings may not apply to women who experience ongoing occupational problems. Noting the implications for infant health, the investigators conclude, "Public health efforts to reduce the incidence of low birth weight might usefully target women experiencing such underemployment events for special preventive interventions."—S. London

REFERENCE

1. Dooley D and Prause J, Birth weight and mothers' adverse employment change, Journal of Health and Social Behavior, 2005, 46(2):141-155.