Leave it to medical dramas to highlight some of the rarest health conditions out there (Grey's Anatomy or ER, anyone?). While some of the issues these shows highlight seem far-fetched, many are based in reality, as in, healthcare experts have seen cases of it before. One such condition? Getting pregnant while you're already pregnant. It's a phenomenon called superfetation¹, and while extremely rate—a 2008 study recorded only 10 cases worldwide²—it is totally real.

Curious as to how this happens? Read on to learn how double pregnancies occur and how to potentially prevent them.

Why superfetation pregnancy happens

Before we get into how superfetation happens, let's cover some of the basics of conception. First, an egg gets released from your ovary (female reproductive organ) during ovulation—a few days during the menstrual cycle when you're most fertile. Then, if you have unprotected sex (i.e., without a condom or birth control method) with a partner with a penis, their sperm can fertilize your dropped egg, and voilá—conception occurs. The combined sperm and egg will form an embryo, which implants into the lining of the uterus and grows into a fetus.

With superfetation, this entire process happens twice, meaning the second embryo joins the first one in your uterus.

For most people, ovulation stops while pregnant, meaning the uterus won't allow another embryo to implant when there's already one in there, says Allison K. Rodgers, MD, a board-certified OB/GYN, reproductive endocrinologist, and director of education at the Fertility Centers of Illinois. In fact, the hormonal changes your body goes through during pregnancy almost ensures this scenario doesn't happen. These changes including the following:

• Your hormones stop your ovaries³ from releasing another egg, making ovulation super unlikely.
• A mucus plug also forms in your cervix, blocking sperm from reaching your uterus.
• Even if sperm somehow got through, your hormones would prevent⁴ a second embryo from implanting in your uterine lining.

When superfetation occurs, a new pregnancy circumvents these natural safeguards. But medical experts are still not exactly sure why or how this ends up happening.

How common is it to get pregnant while pregnant?

“It is exceedingly rare,” Dr. Rodgers says. Only about 10 or so cases of superfetation have ever been recorded in humans, though it is more common in other species like rodents and other small mammals.

“Some even question if it is a real phenomenon in humans,” Dr. Rodgers says. A more likely scenario is that superfetation gets confused with another reproductive issue. For example, if you’re pregnant with twins and one is developing slower, it might seem like the second implanted later, she says. But in reality, they both implanted at the same time—one’s just growing at a different pace.

At the end of the day, you have “less than one in 100 million” chance of experiencing superfetation, Dr. Rodgers says. Put simply, your odds are practically zero. (But it's still fascinating to learn about!)

Are more people more likely to experience it?

Because only a handful of superfetation has been documented, “it’s hard to determine” whether there’s a pattern of risk factors, Dr. Rodgers says. That said, many instances of superfetation have involved some form of assistive reproductive technology (ART), such as intrauterine insemination⁵ (IUI) or in vitro fertilization (IVF). In other words, people receiving fertility treatments may be unaware that they are already pregnant, resulting in two (or multiple) embryos at once.

That said, medical researchers cannot completely confirm the link between superfetation and fertility treatments. Even with ART, your chances of superfetation are extremely low, per Dr. Rodgers. You'll just want to make sure you're checking in with your fertility specialist regularly and taking pregnancy tests to ensure you're not already pregnant before treatments.

Another (unconfirmed) theory suggests that an ectopic pregnancy—when a fertilized egg implants outside of the uterus, usually in the fallopian tube—could slightly raise the risk of superfetation. However, more medical research is needed to confirm this connection. Dr. Rodgers does point to one case study⁶ of superfetation which reported an ectopic pregnancy at 10 weeks and a "normal" pregnancy at 5 weeks. But again, this is extremely rare and the exact biological processes behind it are largely unknown.

If you're concerned about your risk of superfetation or pregnancy complications with ART, let your healthcare provider know. They can help offer guidance and support.

Does it mean you’re having twins?

Technically, no, superfetation does not mean you're having twins. Even though both embryos grow together in the same uterus and will probably be born at the same time, they weren’t conceived in the same menstrual cycle (if they had been, that's a phenomenon called superfecundation). Since each fetus was conceived during separate menstrual cycles, they are at different stages of pregnancy, with one being a few weeks older than the other.

Are there any potential risks or complications?

According to Dr. Rodgers, there are a few potential complications and risks with superfetation. For one, "when the uterus is in labor," both fetuses would be delivered. But the problem is, if one fetus develops faster than the other, the younger one might be born prematurely, Dr Rodgers says.

When an infant is born early—before week 37 of pregnancy—it doesn't have enough time to fully develop certain organs (like the heart and lungs, for example), which can cause serious health problems and developmental delays. If this is the case, the premature infant would need special care in the neonatal intensive care unit (NICU) at the hospital.

In the cases of superfetation, a healthcare provider may recommend a planned Cesarean section (C-section) birth, rather than vaginal birth, to increase the chances of a smooth delivery for both infants.

How to prevent pregnancy while pregnant

It's not likely that you'll have to worry about superfetation when you're expecting. “This is not a typical thing,” Dr. Rodgers says. “It’s not in books, and we don’t learn about it in medical school,” she adds. In other words, superfetation is so rare that, while interesting to learn about, it won't likely happen to you. (You already have enough going on when you're expecting and prepping to be a new parent!)

That said, if you're feeling uneasy about having sex while pregnant, or are worried about other potential pregnancy complications, reach out to your healthcare provider for advice and support. They can help come up with ways to have more comfortable, and safe, sex while pregnant—such as abstaining from penetration or wearing condoms. Providers can also run tests to ensure you're receiving proper care for any pregnancy side effects you may have.