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-W. Bradford Wilcox
Associate Professor of Sociology, University of Virginia
With good reason, public-health officials express concern about babies born too early for their bodies to have developed fully. A large new study of these babies, however, concludes that the mothers giving birth to such imperiled offspring are disproportionately unmarried, regardless of the age of the maternal group in view.
Affiliated with Oxford, Columbia, and the University of Michigan, researchers began their inquiry into preterm birth (PTB) fully aware that PTB constitutes “an important contributor to serious neonatal morbidity, severe childhood disability, and perinatal and neonatal mortality.” Even before assessing their own data, the researchers knew that though only about “6-10% of all births in wealthy countries are preterm, more than two-thirds of all neonatal deaths in this context are [among] preterm infants.”
The investigators also began their study conscious of previous research showing that “maternal marriage is protective against preterm birth (PTB), while advanced maternal age is associated with increased PTB risk.” The question the researchers therefore asked themselves was how marital status affected PTB risk across the maternal life course. Does wedlock reduce the risk of PTB among mothers of “advanced maternal age” less—or more—than it reduces the risk of PTB among very young mothers?
To assess the relationship between maternal marital status and maternal age, the researchers analyzed the data for 1,529,159 single births in Michigan between 1995 and 2006. In these data, the researchers discerned a strong—and fully expected—overall correlation between maternal marital status and risk of preterm birth: though preterm births accounted for only 9.2% of the total births in the sample, the researchers calculate that “among unmarried mothers, 12.0% of births were preterm, which was significantly higher (p < 0.001).” By using a sophisticated multivariable logistic regression model that takes into account background variables such as maternal education, race, and number of previous children, the researchers determined that the risk of PTB actually runs almost 40% higher among unmarried mothers than it does among married peers (Odds Ratio of 1.38).
Through further analysis, the researchers show that the protective effect of wedlock is not the same for all ages but is most pronounced among older mothers. The numbers indicate that “among mothers younger than 20 years, the odds ratio of PTB among unmarried mothers relative to married mothers was 1.17, increasing to 1.70, by the age of 36-40 years, and then decreasing slightly to 1.58, among mothers aged 41 years or older.”
The researchers advance three plausible but somewhat speculative explanations of the pattern they have uncovered linking marital status to marital age in predicting preterm birth. One possible explanation of this pattern might simply be that of “marital selection.” That is, “healthier women are more likely to marry and stay married than their less healthy counterparts.” What is more, it is quite possible that “as mothers age, the pool of unmarried mothers declines in health as the healthiest at each age group have a higher probability of becoming married. Therefore, marital selection may, in part, explain the overall trend of increasing PTB risk among unmarried mothers relative to married mothers with age.”
But an analytical focus on “psychosocial stress” yields a quite different account of the linkage between maternal marital status and maternal age as predictors of preterm birth. In this second possible explanation of this linkage, the researchers suggest that “the advantage of married mothers with respect to PTB risk is a reflection of the role of marriage in buffering social stressors that may increase in volume or become more difficult to cope with as a woman ages. . . . It follows that marriage would become more protective against PTB risk with increasing age.”
Pursuing a related but not identical line of thought, the researchers posit a somewhat more complex connection between maternal marital status and maternal age in determining risk of preterm birth. In this line of reasoning, what is most important in determining such risk is “cumulative exposure to stressors,” or, “the ‘weathering’” effect of stress over time. The researchers find it “plausible that older married mothers would have benefited from potential health protective corollaries of marriage, such as financial security, social support, and better access to healthcare, for a longer cumulative time then younger married mothers. . . .Thus, age would magnify health differences owing to relative disparities in financial security, social support, and access to healthcare between married and unmarried mothers, thereby explaining why marriage may be more protective among older vs. younger mothers.”
However epidemiologists do finally explain the way marital status predicts preterm births among older mothers, this new study hardly augurs well for infant well-being at a time when unprecedented numbers of older women are bearing children out of wedlock.
(Abdulrahman M. El-Sayed, Melissa Tracy, and Sandro Galea, “Life Course Variation in the Relation between Maternal Marital Status and Preterm Birth,” Annals of Epidemiology 22.3 : 168-174.)