Everyone has heard the story of the woman who got pregnant while she was pregnant, right? It’s right up there with the story about the woman who gave birth to twins with different dads and the lady who didn’t know she was pregnant until she went into labor. It’s easy to pass these stories off as urban legends, but scientists have determined that at least one of them is true: not only does science say that a woman can get pregnant when she’s already pregnant, there are stories to prove it.

How does it happen?

The situation is rare, but getting pregnant while pregnant has a name: superfetation. Superfetation, or the process of two conceptions, isn’t actually that complicated. When a woman gets pregnant, her body typically stops releasing eggs from her ovaries. Her body recognizes that one egg has already been fertilized and starts focusing on the growth and development of that fetus. However, in some unusual cases, that doesn’t happen and the eggs just keep coming. And where there are eggs, there is potential for fertilization. If the ovaries release an egg and it gets fertilized, whether or not there is already a bun in the oven, you get superfetation and double babies.

Why is superfetation so unlikely?

The definition makes it sound so simple that it’s a wonder that more people don’t get pregnant while they are already expecting. However, the human body is pretty incredible, and it does everything it can to prevent double pregnancy from happening. Not only do ovaries usually stop releasing eggs during pregnancy, but sperm also has to be able to reach the second egg, which is much trickier during pregnancy than during other times in a woman’s life. Certain hormones skyrocket during pregnancy that creates a mucus barrier in the cervix that protects the fetus while preventing outside influences (like sperm) from entering and potentially fertilizing. Even if an egg made it out of the ovaries and the sperm made it past all the mucus, that fertilized egg would still have to find a viable place in the uterus to implant and live, and when there is already another fetus, the chances of there being enough space is pretty unlikely.

What are the side effects of getting pregnant while pregnant?

Since superfetation is so unlikely, there aren’t a lot of documented side effects of the condition. The fetuses will probably be different sizes, because they’re different ages, and they might even have different blood types. The most dangerous side effect of superfetation doesn’t occur until birth: it is very likely that the younger fetus will be born prematurely. Depending on how far into the first pregnancy the mother was when she got pregnant again, the second baby could suffer from breathing problems, bleeding in the brain, low birth weight, or a host of other premature-baby issues.

Mom also might have a difficult pregnancy if there is more than one baby. Women who carry multiples (whether it’s from superfetation, the release and fertilization of two eggs at the same time, or IVF) are at a higher risk of developing gestational diabetes and having high blood pressure. Because the babies might have to be born at different times, post-childbirth recovery might take longer, too.

But has it really happened?

Superfetation is unlikely, but that doesn’t mean it’s impossible. There have been a handful of scientifically documented cases of superfetation over the years, and each case is just as unique as the condition itself.

Kate Hill and her husband had been trying to get pregnant for a while. She was carefully counting the days of her menstrual cycle and was undergoing hormone treatment to help. When Hill discovered she was pregnant, she was thrilled, but a couple of weeks later when she realized that she was pregnant with not one but two babies she was astounded. Not only had her body released a second egg for fertilization after she was already pregnant, but sperm from the encounter that created the first fetus stuck around long enough to fertilize it. Doctors determine that Hill’s babies were conceived just 10 days apart and were thus able to be born safely on the same day. Because Hill and her doctors had been paying so much attention to her fertility, they were able to make the surprising declaration of superfetation.

In 1999, a woman discovered she was pregnant and went to the doctor for her first ultrasound. Not only did the scan show two fetuses, but the doctor was also clearly able to see that the two fetuses were drastically different sizes. After much deliberation and more ultrasound scans, the doctor determined that it was superfetation. The pregnancy proceeded as normal and two healthy babies were born, one at approximately 39 weeks of development and the other at approximately 35.

Superfetation is slightly more likely in women undergoing hormone treatments or IVF procedures as seen in a woman who got pregnant in 2005. She was pregnant with twins from an IVF procedure, but when she went in for an ultrasound around 5 months into her pregnancy, the doctor saw a third smaller fetus. She had apparently gotten pregnant with twins from IVF and then got pregnant with another baby naturally.

Although superfetation is scientifically possible, it is an incredibly unlikely phenomenon. If you’re already pregnant, you’re probably not going to get pregnant during your pregnancy, and that’s probably a good thing.