How Does Nipple Stimulation Induce Labor? A Clinical Trial Looks for Clues

Nipple stimulation is a natural way for pregnant mothers to try to trigger uterine contractions and begin the labor process. A common belief among scientists has been that this process stimulate the release of oxytocin, the naturally occurring hormone that causes the uterus to contract.

However, a new study finds that while nipple stimulation appears to increase uterine contractions, it does not do so through a sustained measurable increase in mothers’ circulating oxytocin concentration. While this study (more details below) was small and preliminary, it raises questions about other factors that might increase contractions.

Inductions of labor are on the rise. More women are entering pregnancy at advanced ages and/or have co-morbidities such as hypertension or diabetes that make it safer for the mother to have an accelerated delivery to reduce risks such as eclampsia. However, while inducing labor can be medically necessary, it may also increase the risk of certain complications like postpartum hemorrhage.

At Yale School of Medicine, Molly McAdow, MD, PhD, assistant professor of obstetrics, gynecology & reproductive sciences, is trying to better understand the underlying mechanisms through which natural labor induction such as nipple stimulation can create uterine contractions. She hopes these new insights will help providers learn how to induce labor in a way that is safer and more similar to natural labor. She published the findings from her latest study in the American Journal of Obstetrics & Gynecology MFM on February 6.

“Our study suggests that oxytocin is only one piece of labor induction,” says McAdow, who was first author of the paper. “Nipple stimulation causing uterine contractions without a sustained increase in oxytocin suggests there are other mechanisms at play.”

Research has shown that nipple stimulation causes the release of endogenous [within the body] oxytocin, but in a pulsatile fashion. “It creates this rapid surge of oxytocin, and then it drops immediately,” explains McAdow. Nipple stimulation may be conducted through self-massage using one’s fingers. Women may also choose to use a breast pump.

Meanwhile, patients who receive synthetic oxytocin (Pitocin)—the standard of care for labor induction—through an IV experience a spike in the hormone that remains stable throughout labor. This prolonged exposure to the synthetic hormone is not only time-consuming and resource-intensive, but it also comes with some risk. The drug can make the muscle cells in the uterus become less responsive to endogenous oxytocin, which can increase a woman’s risk of postpartum hemorrhage following the delivery.

And there may be other effects that researchers still don’t fully understand, says McAdow. “Our study is helping us identify the differences between the typical induction of labor and this alternative approach, which may be a way of inducing the spontaneous mechanisms of natural labor.”

The new study is part of the larger Stimulation To Induce Mothers Study (STIM), an ongoing, multi-center randomized controlled trial designed to compare the efficacy of nipple stimulation with that of Pitocin. McAdow is the site principal investigator of the STIM study at Yale. The study is randomly selecting over 500 women to receive either nipple stimulation or IV-delivered Pitocin for the first two hours of labor induction. After the two hours, women in the former cohort have the option to switch to Pitocin or continue with the stimulation. Meanwhile, researchers are studying factors including time to delivery, method of delivery, breastfeeding success rates, and patient satisfaction.

For a small subset of 22 participants, the researchers also collected blood samples serially in the early part of labor and measured oxytocin levels in their blood. The latest publication is based on this subset.

The team found that after two hours, both groups experienced uterine contractions at similar frequencies. Patients who received Pitocin showed a significant rise in their circulating oxytocin concentration. However, those who received nipple stimulation showed no measurable increase in this hormone despite having regular contractions. “We think that nipple stimulation is causing a release of oxytocin that is leading to a lot more changes we don’t understand yet,” McAdow explains.

In her ongoing trial, McAdow hopes to identify other potential biomarkers that differ between patients in both cohorts. Her team is also collecting blood samples from patients early in spontaneous labor to see if nipple stimulation is inducing any other biochemical factors that are also triggered during natural labor.

Although the team’s observations so far are intriguing, there still needs to be more research to produce findings with statistical significance. “This is all really exciting, promising data,” says McAdow. “But we need to complete our clinical trial before we draw definitive conclusions.”