Babies are born earlier and smaller when males are scarce

February 11, 2013
Written By:
Laurel Thomas

ANN ARBOR—In communities where Dad is more likely to be missing from the picture, more babies are born prematurely and of lower weight, according to a University of Michigan researcher.

In a study reported in the American Journal of Human Biology, Daniel Kruger, research assistant professor at the U-M School of Public Health, found that when males are scarce, it often negatively impacts birth outcomes—possibly from subconscious regulation of maternal investment in the pregnancy.

“I don’t think any woman is consciously thinking, ‘I should have a premature baby,'” said Kruger, a faculty associate with the Population Studies Center at U-M’s Institute for Social Research. “It is likely a nonconscious system regulated by hormones, etc., that is influenced by both conscious and nonconscious processes.”

Kruger said the outcome is related to investment tradeoffs with deep histories.

“This system evolved over many, many generations before modern times when infant and child mortality rates were much higher,” he said. “There was a very real risk that when conditions were poor, their infant or child would die.

“It would make sense for the mothers to conserve their resources so that they would have more chance of having another child when times were better. Again, this is not a conscious decision, but a system that was effective in the very long run.”

Kruger and students Jillian Clark and Sarah Vanas from U-M’s College of Literature, Sciences, and the Arts combined year 2000 birth records aggregated by county from the U.S. Centers for Disease Control and Prevention with 2000 U.S. Decennial Census data.

Across the 450 counties, they calculated the proportions that were premature, defined as less than 37 weeks gestation, and those with birth weight below 5.5 pounds. They developed a path model that took into account the ratio of men to women ages 18-64, single-mother households as a proportion of families with children, socioeconomic status, the degree of male scarcity and other factors.

Kruger said the public health community has worked to support single mothers in their pregnancies with interventions that follow a standard medical model but now should also work to “assess the level of support that women expect to have from their partners as well as family members and others in their social networks.”

“If possible, we should increase men’s involvement and support during pregnancy and reinforce the expectation that he will be around to help raise the child,” he said. “This does not just have to be financial support. It can be spending time with the child, taking care of him/her, teaching him/her skills, etc.”


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