Superfetation is a rare event that involves getting pregnant a second time while you're already pregnant. It's so uncommon that cases of superfetation often make headlines. Your body does a good job preventing subsequent pregnancies once an embryo is developing inside your uterus.

While it sounds impossible, a phenomenon known as superfetation allows a second pregnancy to occur even when a woman is already pregnant. It is extremely rare in humans, with fewer than 10 to 12 confirmed cases in medical history. Typically, when a woman becomes pregnant, her hormones change to stop the ovaries from releasing more eggs and her cervix forms a mucus plug to block sperm. In superfetation, these natural barriers fail: a second egg is released, fertilized, and successfully implants in the uterus alongside the existing embryo. This results in "twins" of different gestational ages, meaning one baby is more developed than the other. Most modern cases have occurred during assisted reproductive treatments (like IVF) where hormonal cycles were manipulated. While common in animals like hares and badgers, in humans, it remains a biological anomaly that usually results in both babies being delivered at the same time, often with the younger baby being born prematurely.