Breastfeeding During Pregnancy and Risk of Miscarriage
by Anne Eglash MD, IBCLC, FABM
Until recently there has been very little data on the risk of breastfeeding during pregnancy. Some have hypothesized that the uterus down-regulates its oxytocin receptors during pregnancy, so that the uterus is not as likely to contract during oxytocin release due to sexual activity, nipple stimulation, or breastfeeding.
Clinical Question of the Week #54 addressed a systematic review that did not have any evidence indicating an increased risk of miscarriage due to breastfeeding during pregnancy.
The article for this week is based on data collected on 10,661 pregnancies in the U.S. from the National Survey of Family Growth, covering various years between 2002-2015. The authors focused on women with singleton births who breastfed, who had no barriers to breastfeeding during a subsequent pregnancy. They evaluated answers to questions regarding breastfeeding during a subsequent pregnancy, the interpregnancy interval, whether the mothers exclusively breastfed during pregnancy or also offered complementary foods to the nursing child. Demographics such as age, education, race/ethnicity, pregnancy intention, and history of miscarriages were evaluated as well.
The authors described the frequency of breastfeeding during pregnancy as 6% of the total time at risk (person-months).
- Most mothers were already giving complementary foods when they were nursing during pregnancy.
- Mothers who breastfed during pregnancy, and were also giving complementary feedings to their children did not have a higher risk of miscarriage compared to pregnant women who were not breastfeeding.
- Mothers who breastfed exclusively during pregnancy had a significantly higher risk of miscarriage vs mothers who breastfed during pregnancy but also gave complementary foods to their children.
- Breastfeeding during pregnancy was more common for mothers who became pregnant earlier than planned, vs those who viewed the pregnancy as occurring at the right time.
- Breastfeeding during pregnancy was found to be highest among teenage mothers.
- Breastfeeding-during-pregnancy rates were highest among the shortest interpregnancy interval, less than 12 months.
See the Answer
Breast-feeding rates and durations have been increasing among U.S. women in recent decades. As a result, women may be more likely to practice breast-feeding during pregnancy (BDP), which has been hypothesized to increase the risk of miscarriage, yet there has been little research into the issue.
Data on 10,661 pregnancies from several waves of the National Survey of Family Growth, covering the years 2002–2015, were used to calculate unadjusted miscarriage rates according to BDP status. Multivariate Cox pro¬portional hazards models were employed to investigate the association between BDP and the risk of miscarriage.
BDP was practiced for 6% of the total time at risk of miscarriage. The miscarriage rate was higher when mothers exclusively breast-fed during pregnancy (35%) than when they practiced either complementary BDP (i.e., the child also consumed other food) or did not breast-feed (14% and 15%, respectively). After adjustment for maternal and pregnancy characteristics, the risk of miscarriage was greater when mothers exclusively breast-fed than when mothers did not breast-feed (hazard ratio, 3.9), but no increased risk was found with complementary BDP. The miscarriage risk during exclusive BDP was similar to that for women who conceived when they were 40 or older (3.2).
Exclusive BDP is associated with an elevated risk of miscarriage, but it remains unclear whether and how the practice is associated with health outcomes for the mother and breast-fed child. Research is needed to further explore these outcomes to inform recommendations regarding BDP.
I believe that this is the first set of U.S. data on the frequency of breastfeeding during pregnancy, and the first study to show a link between exclusive breastfeeding during pregnancy and miscarriage. The biggest limitation of this study is that is based on recall of breastfeeding and pregnancy details.
The study controlled for other risk factors for miscarriage such as maternal age, education, history of miscarriages, but they did not control for socioeconomic level. Interestingly, the risk of miscarriage was much higher for exclusively breastfeeding than for breastfeeding with complementary feeding. The authors surmise that this may represent a nutritional problem for mothers who are exclusively breastfeeding. The systematic review on breastfeeding during pregnancy that was addressed in Clinical Question of the Week #54 also raised evidence that the newborn may be at risk for low birth weight and mothers at risk for nutritional compromise, with most of that data coming from non- U.S. countries. The authors of this week’s article suggest that we need more prospective data, e.g. following a large cohort of mothers over time so that we can be more exact about their breastfeeding behaviors and identification of miscarriages. In the meantime, it would be reasonable to encourage and provide longitudinal nutritional management for all pregnant mothers who are breastfeeding.