Exclusive breastfeeding during pregnancy might increase the risk of miscarriage, study suggests

Published: 2019-09-24

Does the risk of miscarriage increase when a mother breastfeeds while pregnant with a subsequent child? Yes, it might – if the breastfeeding is done exclusively. Complimentary breastfeeding does not increase the risk of miscarriage at all, according to a new study by LUSEM, published in Perspectives on Sexual and Reproductive Health. There is a need for more research before any general recommendations can be made.

Photo: Jordan Whitt/Unsplash

“This study questions the safety of practising exclusive breastfeeding during pregnancy. The results suggest that exclusive breastfeeding during pregnancy may be more strongly correlated with miscarriage risk than a woman’s history of pregnancy loss or becoming pregnant at age 40 or older – both of which are well-known predictors of miscarriage,” says Joseph Molitoris, demographer and researcher at Lund University School of Economics and Management.

Joseph Molitoris has conducted a study with data from the U.S. National Survey of Family Growth to investigate whether breastfeeding during pregnancy is associated with the risk of miscarriage in the first 20 weeks of pregnancy. Data on 10,661 pregnancies – covering the years 2002–2015 – were used. The study takes into account whether the breastfeeding was practised complementarily (i.e., the child also consumed other food) or exclusively (i.e., mother’s milk was the child’s only source of nutrition). About 6% of US women practised any form of breastfeeding during pregnancy. Only about 0.4% of women practised it exclusively.

“The share practising exclusive breastfeeding during pregnancy was small, but their risk of miscarriage was very high. A pregnancy in which a woman was practising exclusive breastfeeding had almost four times higher risk of ending in a miscarriage than a pregnancy in which the woman was not breastfeeding at all,” Joseph Molitoris says.

The study found that the miscarriage rate was higher when mothers exclusively breastfed during pregnancy (35%) than when they practised either complementary breastfeeding during pregnancy (14%) or did not breastfeed (15%). After adjustment for maternal and pregnancy characteristics, the risk of miscarriage was greater when mothers exclusively breastfed than when mothers did not breastfeed (hazard ratio, 3.9).

Pregnancies that overlapped with complementary breastfeeding, however, were not at an elevated risk of miscarriage, suggesting that the intense physical and nutritional demands of both exclusive breastfeeding and being pregnant may explain the increased miscarriage risk.

Joseph Molitoris stresses the importance of future research on this topic before any new recommendations about breastfeeding and pregnancy can be made.

”This is just one study, and we need much more information before we make new recommendations. For example, women practising breastfeeding during pregnancy also tended to be below age 20, their pregnancies were more likely to be unwanted or mistimed, and their previous children tended to have lower birth weights. More research is also needed to understand the mechanisms that are linking breastfeeding during pregnancy to miscarriage. My study only provided an association between the two, but the mechanism, whether it is hormonal or nutritional or something else, will determine what the proper course of action is if this association remains robust.”

Breastfeeding rates and durations have been increasing among U.S. women in recent decades. As a result, more women may become pregnant whilst still breastfeeding, which has been hypothesized to increase the risk of miscarriage, yet there has been little research into the issue.

The current study is the first to investigate this topic in the United States, and one of the strengths of the study is that it is one of few to make use of nationally representative data. Among the limitations of the study, Joseph Molitoris counts the fact that the data came from retrospective pregnancy histories, the details of which may sometimes be difficult for respondents to recall accurately. Also, the study could not consider the intensity of breastfeeding, which is likely an important moderating variable. Another limitation is that data on miscarriages have some well-known problems. Among these is the tendency to underreport, whether unintentionally or intentionally, early pregnancy losses. A final limitation is that this study was unable to control for the many unobserved factors that may be correlated with breastfeeding during pregnancy and the risk of miscarriage. Mothers who breastfeed during pregnancy may differ from those who do not in ways that we cannot observe in observational data.

Previous studies that have investigated the relationship between breastfeeding during pregnancy and the risk of miscarriage have generally found no statistically significant relationship. But the current literature suffers from several shortcomings, according to Joseph Molitoris. All of these studies were conducted using small, clinical or local samples, and it is, therefore, unclear whether the findings apply to general populations.

No study of breastfeeding during pregnancy before this has differentiated between complementary and exclusive breastfeeding. This is an important distinction, as one would expect that exclusive breastfeeding would be far more demanding on a mother in terms of nutrition and stress than complementary breastfeeding.

“These findings are particularly relevant for women who conceive at a young age or following a short interpregnancy interval, both of which may be risk factors for miscarriage and may, therefore, serve to increase the risk of pregnancy loss,” Joseph Molitoris says.

Researching the relationship between breastfeeding during pregnancy and miscarriage is challenging though, because many currently available datasets are not well-suited for answering these questions. In order to properly answer this question, a researcher needs detailed data on when pregnancies begin and end, when breastfeeding begins and ends, maternal nutrition, the intensity of breastfeeding, and the outcomes of pregnancies for all of the pregnancies a woman ever experienced. This kind of information is not easy to come by in most large-scale, nationally-representative surveys.

“I think properly addressing some of the shortcomings of my paper really requires a specifically designed observational study that follows women forward over time. This kind of study could minimize some of the problems concerning respondents’ recall and their willingness to omit certain pregnancies from their histories.”

Publication: Breast‐feeding During Pregnancy and the Risk of Miscarriage


Joseph Molitoris

Researcher in Economic Demography at Lund University School of Economics and Management

E-mail: joseph.molitoris@ekh.lu.se

Phone: +46 76 408 8038

Joseph Molitoris in the Lund University Research Portal