It's kind of an old-wives' tale that you can't get pregnant while breastfeeding. In actuality, women can, and they certainly do. "The likelihood of getting pregnant while breastfeeding regularly is low due high levels of the hormone prolactin, which stimulates milk production and discourages ovulation," says Sharon Phelan, M.D., a professor of obstetrics and gynecology at the University of New Mexico Health Science Center School of Medicine. "But it's not a 100-percent effective birth control method, because once you begin menstruating regularly again—usually 3 to 4 months after giving birth—chances are you're ovulating normally again, too."
There's a lot of controversy around whether or not you should continue to feed the baby in your arms when you've got another one in the oven. "Part of it is societal pressure," says Phelan. In the United States, women tend to breastfeed on a set schedule, and they're encouraged by outside factors to wean babies while they're still in diapers. "And then there's the physical aspect of taking in enough calories to sustain a newly growing baby in addition to producing enough milk for the other one," she says. In other words, you could end up in a state of constant hunger and exhaustion—which is why you'll need to eat plenty of nutrient-dense foods. Other symptoms may include worsened morning sickness, nipple soreness, and breast tenderness.
If you do find yourself pregnant and nursing, you won't see a major change in your milk supply initially says Patrick Duff, M.D., an ob-gyn at the University of Florida, Gainesville. But that doesn't mean a drop-off isn't coming. As the pregnancy hormones estrogen and progestin ramp up to encourage the growth of the new baby, the amount of milk-producing prolactin being released will significantly go down. "That's when women will notice a major decrease in milk output, and many simply won't be able to continue nursing," says Patrick Duff, M.D., an ob-gyn at the University of Florida, Gainesville. "Still, as long as there's adequate milk for the toddler, it's generally okay to keep at it." Expect the decrease in milk supply to happen around 4 or 5 months.
Duff says breastfeeding while pregnant it won't harm either baby. However, there is a hormone response that could be a cause for concern. "When you breastfeed, your body releases the hormone oxytocin, which relaxes muscles in your breast and encourages milk to flow through the nipples," says Phelan. That oxytocin isn't localized to your chest. In fact, it circulates throughout your body and pretty much relaxes everything—including your uterus, which could lead to premature contractions. These are generally not an issue, but “if you have any concerns about early delivery, then it might be a good idea to start weaning off your current baby," says Phelan.
Your doctor may encourage weaning in certain situations; for example, if you’re experiencing unusual bleeding, or if you have a high risk of premature labor. Always speak to a professional about any questions, concerns, or unusual symptoms you may have.