Beyond marital status: relationship type and duration and the risk of low birth weight

Unmarried women have higher rates of low birth weight than married women. However, assumptions that unmarried women are uniformly at a disadvantage may be unfounded. A woman's relationship characteristics may be more relevant for infant health than her formal marital status.

Data from the 1995 National Survey of Family Growth were used to analyze associations between relationship characteristics and low birth weight among U.S. women aged 15-44 with a recent singleton live birth. Unadjusted odds ratios were generated to indicate the crude effects of independent variables, including relationship type and relationship duration at the time of conception. Multiple logistic regressions were performed to assess the impact of relationship variables on the likelihood of low birth weight, taking into account the effects of other covariates.

In multivariate models of all women and non-Hispanic black women, relationship type and duration were not associated with low birth weight. However, low birth weight was almost six times as likely among Hispanic women in nonmarital, noncohabiting relationships as among those who were married. Surprisingly, among non-Hispanic white women, low birth weight was less likely among those in nonmarital, noncohabiting relationships than among those who were married. Unexpected associations also were found among low birth weight, race and ethnicity, and relationship duration: Low birth weight was more likely among non-Hispanic white women in relationships of from five to 10 years in length than among those in relationships of longer than 10 years and less likely among Hispanic women in relationships of one year or less than among those in a relationship for more than 10 years.

Although unmarried women in the United States have higher rates of low birth weight than married women, many unmarried women are at no greater risk of low birth weight than their married counterparts. The findings confirm the need to consider the characteristics of relationships when examining the association of mother's "union status" and birth outcomes.