The physiological cost of reproduction for rich and poor across 65 countries

One of the fundamental tradeoffs posited in life history theory is between storing energy for future reproduction versus spending that energy on current reproduction. However, past studies have shown variable and sometimes contradictory effects of reproduction on energy stores among women.

To examine how varying economic resources can account for these diverse findings, we applied mixed models to Demographic and Health Survey (DHS) data from 187,848 nulliparous and primiparous women of reproductive age (20-34 years) in 65 countries varying widely in economic resources. Using this approach, we tracked average trajectories of body mass through pregnancy and the post-partum period, and assessed how these trajectories varied by household wealth and breastfeeding.

In all four wealth categories, sustained breastfeeding posed a substantial tradeoff with energy stores, reducing post-partum BMI by 0.5 to 1.0 kg m(-2) relative to non-breastfeeding women. However, among the wealthiest households (>6,400 USD per capita), this deficit was buffered substantially by greater pre-partum weight gain (+1.1 kg m(-2) compared to women from the poorest households).

These findings show how the level of economic resources can systematically and profoundly shape a physiological tradeoff in reproduction, and can help account for past contradictory findings. More broadly, these results illustrate how integrating economic and energetic resources in a common framework can help clarify the apparently disparate weight-related outcomes of fertility in different countries.