You're likely familiar with the basics of how a person gets pregnant: An egg is released from an ovary; sperm fertilizes the egg; the fertilized egg attaches to the uterine lining, where it eventually becomes a fetus. But what if the process occurs a second time, after one egg has already implanted in the uterus?
This phenomenon is called superfetation, and it happens when two or more fetuses at different stages of development coexist as the result of a second ovulation-fertilization-implantation process. It's known to occur in some mammals such as rodents and rabbits, according to a winter 2021 case report published in the Ochsner Journal.
Superfetation is highly unusual in humans. "It's incredibly rare, and there aren't many reports on this," said Christine Greves, MD, an OB-GYN in Orlando, Florida. "It's so rare, in fact, that I can't even give you a statistic on it."
Fewer than 10 cases of the phenomenon had been described in scientific literature, as of June 2021, when a case report on possible superfetation was published in Obstetrics and Gynaecology Cases - Reviews. Scientists had hypothesized that these cases are only possible when women have undergone an assisted reproductive technology (ART) treatment—any type of fertility treatment involving the handling of eggs or embryos, according to the Centers for Disease Control and Prevention (CDC).
Superfetation is rare because, once a person is pregnant, changes in their hormones typically prevent their body from continuing to ovulate, Dr. Greves explained. But if a person does ovulate again—and if that egg becomes fertilized and implants in the uterus—they can carry a double pregnancy.
The reasons why a person might ovulate while they're already pregnant weren't well studied, as of April 2022. "Superfetation goes against what we know medically about getting pregnant spontaneously," said Sherry Ross, MD, an OB-GYN in Santa Monica, California. "The truth is not all medical phenomena have a logical explanation."
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In short, it's very difficult. "Superfetation is controversial and hard to prove," Dr. Greves said.
For example, in a case report published in 1999 in the journal Ultrasound in Obstetrics & Gynecology, researchers identified a four-week discordance between two fetuses. In other words, one fetus appeared to be four weeks older than the other, which suggested a superfetation pregnancy. But fetuses that appear to have different gestational ages aren't necessarily proof of superfetation.
There is such a thing as twin-twin discordance, Dr. Greves pointed out, which means the fetuses are growing at different rates inside the uterus. One fetus may be getting more nutrients than the other, and therefore appear bigger (or older) in an ultrasound. "If that's the case, it could look like superfetation, but it wouldn't necessarily be due to that," Dr. Greves said.
If a person who is pregnant has an early ultrasound that reveals only one fetus, and a subsequent ultrasound reveals a second fetus—that could indicate superfetation has occurred. But again, the evidence isn't definitive, since the healthcare professional may have simply missed the second fetus the first time around.
Superfecundation is another rare pregnancy-related phenomenon. It happens when children born together have the same mother, but not the same fathers. That's why it's also known as heteropaternal superfecundation—from different fathers.
There were only about 18 documented cases of superfecundation, as of 2018, according to a 2021 paper published in the journal Forensic Science International. The paper also said it's likely underreported.
The phenomenon occurs when the mother has intercourse with different partners during the same ovulation period. Two ova (or eggs) get released simultaneously, but they're fertilized at different times. This results in half-siblings with the same mother but different fathers, born at the same time—heteropaternal twins.
Superfecundation differs from superfetation because it happens during the same ovulation period, so the mother is not yet pregnant when the two eggs get fertilized. And the siblings develop together. Paternity testing can help determine if the twins have the same father or not.
