July 13, 2021 | Press Release

The Higher Developed a Country, the Weaker the Contraceptive Effect of Breastfeeding

For demographic researchers around the world this is an important finding because long duration of breastfeeding can lower fertility very significantly. © iStockphoto.com/Asia-Pacific Images Studio

What determines how long it takes to resume fecundity after a childbirth for women who breastfeed? Former MPIDR researchers assessed survey data to answer this question in 84 low- and middle-income countries. They found that the contraceptive effect of breastfeeding has sharply decreased in these countries over the past 40 years.

Breastfeeding is a traditional contraceptive method: after a childbirth, the capacity to conceive resumes later in women who breastfeed. For most women in high-income countries this phase only lasts a few weeks to months, as reflected by early resumption of menstruation. But over the past 40 years this time period has also decreased for women in low- and middle-income countries. “We found unexpectedly short durations without menses, and this is not because of reduced durations of breastfeeding”, says Nicolas Todd, former researcher at the Max Planck Institute for Demographic Research (MPIDR) in Rostock, Germany.

For demographic researchers around the world this is an important finding because long duration of breastfeeding can lower fertility very significantly. “There is no need for complex modeling to appreciate that when each birth is followed by an 18-month period free of conception risk, the total number of children a woman will have is much reduced”, says Nicolas Todd.

He and his former MPIDR colleague Mathias Lerch analyzed 2.7 million births in 84 low- and middle-income countries between 1975 and 2019 by assessing survey data from 17 World Fertility Surveys (WFS) and 284 Demographic Health Surveys (DHS). Their study was recently published in the Proceedings of the National Academy of Sciences of the United States of America (PNAS).

Duration correlates with improved living standards

So, what are the reasons behind this earlier resumption of menstruation observed in low- and middle-income countries? The researchers found improved living standards may matter a lot. Characteristics such as a high value for the Human Development Index or better access to electricity and water predicted a reduced association between breastfeeding and duration without menstruation. A likely explanation is that lactation’s heavy metabolic cost plays an important role in postpartum suppression of ovarian activity: when this cost is more easily met because of improved nutrition or reduced physical activity, ovarian function returns earlier.

Reconciling demography and reproductive ecology

These findings contradict previous demographic studies, that found evidence for only a limited environmental influence on the contraceptive effect of breastfeeding.

By contrast, the results are in line with a number of reproductive ecology field studies that went on to measure hormones in small cohorts of women and showed ovarian activity is actually sensitive to context. “We believe we have resolved the long-standing dispute between demography and reproductive ecology on whether breastfeeding duration alone explains the reproductive pause after childbirth”, says Nicolas Todd.