The concern that breastfeeding an older child may cause a miscarriage is a common worry for many expectant mothers. This fear stems from the physiological reality that nursing triggers uterine contractions, which are typically associated with labor. For the vast majority of healthy pregnancies, medical understanding indicates that continuing to breastfeed does not increase the risk of miscarriage or preterm birth. Understanding the science behind the uterine response and knowing the specific, rare situations where caution is advised is important for making an informed decision.
The Science Behind Uterine Activity
The mechanism linking breastfeeding to uterine activity is the hormone oxytocin. Suckling stimulates the release of oxytocin from the posterior pituitary gland, which is responsible for the milk ejection reflex, or “let-down.” This hormone also causes the uterus to contract during labor and immediately postpartum to shrink the organ back to its pre-pregnancy size.
While nursing, the released oxytocin causes the uterus to contract, though these contractions are often mild and may not be noticed during the first and second trimesters. The reason these contractions are generally ineffective at terminating a pregnancy lies in the difference between the pregnant and laboring uterus. The uterine muscle, or myometrium, is largely unresponsive to oxytocin in early pregnancy because it has a very low concentration of oxytocin receptors.
The uterus is naturally designed to remain quiescent, or inactive, during most of gestation. The concentration of oxytocin receptors on the myometrial cells only begins to increase significantly toward the end of the third trimester. This low receptor count acts as a protective shield, preventing the small amount of oxytocin released during nursing from generating contractions strong enough to disrupt a healthy pregnancy.
Medical Consensus on Risk
Based on the physiological understanding of uterine receptor sites, medical organizations generally reassure expectant mothers that nursing is safe in a low-risk pregnancy. Research has consistently shown that continuing to breastfeed through a healthy gestation does not lead to a higher rate of miscarriage or preterm delivery. The vast majority of early pregnancy loss is attributed to chromosomal abnormalities, which are unrelated to external factors like uterine contractions from nursing.
The risk of miscarriage remains consistent whether a woman is nursing or not. While one study found an association between exclusive breastfeeding during pregnancy and a higher risk of miscarriage, this finding did not hold true for mothers who were also offering complementary foods. Researchers suggest that factors other than the mechanical act of nursing, such as a short interval between pregnancies or nutritional status, may contribute to the outcome.
This consensus supports the idea that the body has safeguards in place to protect the developing fetus. The mild contractions that occur are comparable to those experienced during sexual intercourse, which is generally considered safe during uncomplicated pregnancies. For a low-risk mother, the decision to continue nursing is primarily personal, based on comfort and the needs of both the mother and the older child.
When Nursing During Pregnancy Requires Caution
While breastfeeding is typically safe, specific, high-risk medical conditions exist where healthcare providers may advise temporary cessation or full weaning. These recommendations are not based on the idea that nursing causes a problem, but that uterine stimulation could potentially complicate an already fragile pregnancy. This decision is always made on a case-by-case basis after a thorough medical review.
A history of recurrent miscarriage or prior preterm labor is a strong reason to discuss weaning with an obstetrician or midwife. In these scenarios, any additional uterine stimulation from nursing may be considered an undue risk. Immediate cessation of nursing may be recommended if a mother experiences signs of threatened miscarriage, such as unexplained vaginal bleeding or persistent, painful cramping.
Other high-risk scenarios warrant professional evaluation. A diagnosis of placenta previa, where the placenta covers the cervix, may lead to a recommendation to stop nursing due to the increased risk of hemorrhage from uterine stimulation. Conditions like multiple gestation or fetal growth restriction often involve heightened nutritional and physical demands. Continued nursing may be unsustainable or detrimental in these cases. Open communication with the prenatal care provider is the most reliable way to navigate the choice to continue nursing.