When my first child was 24 months, I went to my obstetrician and confirmed what two tests and a healthy dose of nausea suggested: I was pregnant again. I was thrilled to see that pea-sized ball of love on the ultrasound but felt deflated when, at the end of the appointment, my doctor told me I should now stop breastfeeding.

At first I ignored that advice, but felt guilty every time I nursed. Was feeding my toddler really harming baby No. 2?

Breastfeeding has been one of the easiest parts of parenting for me. I didn’t experience much breast tenderness, it helped my daughter fall asleep and I was thankful for the many health benefits for both my child and me. So I was disappointed by the idea of weaning. And I know I’m not alone.

Figures from the Centers for Disease Control and Prevention show that more than 83% of babies are breastfed for at least some time, and about 15% are still nursing at 18 months. This means some parents will experience the same breastfeeding-pregnancy quandary that I did.

Susan Crowe, a clinical professor of obstetrics and gynecology at Stanford University, explains that the advice to stop breastfeeding when you are pregnant was common about 20 years ago, although the practice was not well studied. But “there was a theoretic concern that because there’s a release of oxytocin during breastfeeding,” Crowe said.

Oxytocin, in addition to helping the flow of milk when nursing, also stimulates the uterus to contract during childbirth. The worry was that oxytocin released during nursing “could cause uterine cramping that could lead to miscarriage or preterm delivery,” Crowe said. “It became advice that we were all taught as obstetricians to give without evidence to support it.”

The synthetic form of oxytocin, pitocin, is often used to induce labor.

But Crowe said that naturally released oxytocin during lactation has not been shown to increase the risk of preterm birth, although it can cause cramping and even some contractions. This is in part, she said, because oxytocin receptor sites, the uterine cells that detect oxytocin and cause contractions, are low in number during early pregnancy and increase approximately twelvefold by 37 to 41 weeks.

“Oxytocin receptors are not well-developed in the uterus throughout pregnancy, so that perhaps that is the reason that people can have mild contractions and not go into preterm labor,” she said.

One study, however, in the Journal of Perspectives on Sexual and Reproductive Health found that exclusive (rather than supplemental) breastfeeding was associated with a higher risk of miscarriage. The study found that miscarriage risk for parents who were exclusively breastfeeding was comparable to the risk to people who conceived when they were 40 or older. Jourdan Triebwasser, maternal fetal medicine physician and a clinical assistant professor at the University of Michigan Health, said other factors may be at play in this research, such as pregnancy spacing.

“Short pregnancy intervals can have risks of miscarriage and preterm birth,” she said. “And those with shorter pregnancy intervals are probably more likely to still be exclusively breastfeeding. So that may play into it.”

But some physicians still have concerns about breastfeeding while pregnant, at least in certain scenarios.

“Breastfeeding while pregnant is perfectly fine,” said Tessa B. Scripps, pediatrician and certified lactation counselor at Mount Sinai Kravis Children’s Hospital, although she’s wary of nursing during high-risk pregnancies. She references cases of intense uterine cramping before recommending a cautious approach. “I advise pregnant moms who may be at high risk for preterm delivery to speak with their obstetricians.”

Pregnancy and nursing: more research needed

Part of Scripps’s concern stems from the fact that there still hasn’t been much research on nursing during pregnancies, let alone high-risk pregnancies. “In general,” Triebwasser says, “there’s really limited data on the benefits or risks of breastfeeding during pregnancy.”

The data we do have should be reassuring: A 2017 systemic review of breastfeeding during pregnancy found reported that it “does not affect the way pregnancies end or even birth weights.”

Still, there is truth to the idea that nursing can encourage a baby to come sooner. In one study, pregnant people who had nipple stimulation starting at 38 weeks’ gestation delivered their babies, on average, slightly earlier. Another study found those with nipple stimulation had shorter labors.

“At the end of the third trimester, some people do use nipple stimulation as a way to try to naturally get into labor. So certainly there’s some reason to believe it may affect the uterus late in the pregnancy,” Triebwasser said.

But this doesn’t appear to have connection to preterm birth, and Triebwasser emphasizes that people with healthy pregnancies don’t have reason to worry.

Despite the safety of breastfeeding while pregnant, some parents may decide they would prefer to stop. “The first trimester of pregnancy can be exhausting and hormonal effects can lead people to experience nipple soreness,” Crowe said. “Some people, after experiencing these things, decide that they are ready to wean.”

Other times, the older nursling will naturally decide to stop. Crowe points out that milk supplies will typically decrease during pregnancy. And generally starting around 16 weeks, milk turns back into colostrum, the nutrient-rich first stage of breastmilk.

While colostrum is still good for an older child to drink, Scripps said the changes in milk may inspire a child to wean.

“Often, children who are breastfeeding while moms are pregnant will actually self-wean themselves because they don’t like the taste or because the volume goes down,” Scripps said. “The child may also start to get frustrated or refuse the breast or not gain weight as well.”

Still, Crowe puts emphasis on the power of the parental choice.

“I think that people do need reassurance regarding nursing. They should know they don’t have to stop if they become pregnant,” Crowe said. “But really, this is about supporting people to make the choices that they want to make. Many people are more comfortable continuing with lactation and we want to have that option available to them.”

After learning more about breastfeeding while pregnant, I decided to carry on nursing.

My toddler was happy to continue, and I was thankful to maintain her easy bedtime routine. But as weeks passed, I found myself struggling. I started to have nipple pain and soreness. Plus, I was tired from a rough patch of morning sickness.

I still appreciated the benefits of nursing, but suddenly I wanted to stop – or at least cut back. Now, more than halfway through my pregnancy, I’m still nursing, though just at bedtime.

I’m hoping to continue as long as I can – but I know my firstborn’s breastfeeding days are numbered.