Certain maternal behavioral factors—including receiving prenatal care, low pregnancy-related weight gain and smoking during pregnancy—are significantly associated with low birth weight, according to a 2001 survey of women who gave birth in public hospitals in western Mexico.1 Social, demographic and economic factors—including age, union status, locality size and working during pregnancy—have an indirect effect on low birth weight through their associations with prenatal care.
To identify factors associated with low birth weight, researchers recruited for a 2001 survey women giving birth at one of eight Ministry of Health public hospitals in two western Mexican states. All women who had delivered a low-birth-weight infant (defined as weighing less than 2,500g) were eligible for the survey; researchers randomly selected a sample of mothers of normal-weight infants to participate. Trained female interviewers asked participants while in the hospital's recovery ward about their social and demographic characteristics and certain behavioral factors (i.e., smoking during pregnancy, prenatal care and pregnancy-related weight gain), and whether they had experienced health problems or hospitalization during pregnancy. Logistic regression analysis was used to determine which factors were associated with low birth weight and with prenatal care.
The sample consisted of 565 women: 257 who had delivered a low-birth-weight infant and 308 whose infant was born at a normal weight. Seventy-two percent of respondents were aged 20-34; 21% were younger and 7% were older. The index birth was the first for about one-third of respondents; approximately two-thirds were in a formal union and a similar proportion had had six or more years of education. Fifty-two percent of women lived in a locality with fewer than 100,000 people, and 80% did not work at any point during pregnancy. Between 4% and 17% lacked indoor plumbing or electricity, or lived in houses with dirt floors.
The vast majority of mothers (92%) reported that they had not smoked during pregnancy. Sixty-five percent had gained seven kilograms or more during pregnancy; 17% had gained less and 18% did not know how much weight they had gained. Almost two-thirds (63%) of respondents had received prenatal care during the first trimester of their pregnancy, whereas 25% had received care after the first trimester and 12% had received no prenatal care. Eighty percent of women reported having had health problems during their pregnancy, although only 6% of women required hospitalization.
In a multivariate regression analysis examining which social and demographic factors were associated with low birth weight, only one factor was found to be significant: Women having their first birth were more likely than women of low parity (defined as those aged 18 or older delivering their second child and those aged 25 or older having their third) to have had a low-birth-weight infant (odds ratio, 1.7). When behavioral factors were included in the regression, women who had gained less than seven kilograms during pregnancy and women who did not know how much weight they had gained were more likely than those who had gained seven or more kilograms to have had a low-birth-weight infant (2.3 and 2.0, respectively). In addition, women who smoked were more likely than those who had not (2.1) to have had a low-birth-weight infant; the association with first births remained significant (2.0). In a final model that included measures of maternal health, women who had had problems during pregnancy and those who had been hospitalized had elevated odds of having a low-birth-weight infant (2.1 and 4.2, respectively). Women who began prenatal care after the first trimester were half as likely as those who received no prenatal care to have had a low-birth-weight infant. Being 35 or older and smoking during pregnancy became marginally significant (p<.10), whereas all other associations remained significant.
In another logistic regression, researchers examined which social, demographic and behavioral factors were associated with prenatal care. Women aged 35 or older were more likely than those aged 20-34 (9.5), women for whom the index birth was their first were more likely than those of low parity (3.0) and women who worked during pregnancy were more likely than those who had not (2.5) to have received prenatal care. Women who were younger than 20 were less likely than those aged 20-34 (0.4), women who were not in a union were less likely than those in a formal union (0.3) and women who lived in a locality with 100,000 people or more were less likely than those in a locality with fewer than 100,000 (0.3) to have received prenatal care. In a second model that included a measure for maternal health, women who had had health problems during pregnancy had significantly higher odds of having had prenatal care than did those who had not had such problems. The associations between low birth weight and first births became nonsignificant, and working during pregnancy became marginally significant (p<.10).
In explanation of their finding that only one of the social, demographic and economic factors studied was significantly associated with low birth weight, the authors comment that their sample consisted of "a relatively socioeconomically disadvantaged segment of the population." They add that "the strength of individual-level socioeconomic effect...may be more pronounced with a nationally representative survey sample." Social and demographic factors, however, were significantly associated with prenatal care, which was one of the behavioral factors associated with low birth weight. The authors suggest that programs that work to reduce the rate of low-birth-weight infants should address "improving maternal lifestyle choices by increasing access, utilization and quality of care," while addressing the "intractable socioeconomic disparities that continue to indirectly contribute to the incidence of low birth weight."
1. Frank R et al., Low birth weight in Mexico: new evidence from a multi-site postpartum hospital survey, Salud Púlica de México, 2004, 46(1):23-31.