Heteropaternal (meaning “from different fathers”) superfecundation and heteropaternal superfetation are both extremely rare events. Despite this, they have both been documented in medical records. Both of these processes result in fraternal, or dizygotic, twins, since identical (monozygotic) twins can only come from a single fertilized egg.
Superfecundation or superfetation can both occur naturally, but they can also be the result of certain fertility treatments. In one recorded case, there was an error in an in vitro fertilization (IVF) lab. Equipment that was contaminated with sperm from one man was used again before being cleaned.
The woman receiving the IVF treatment ended up having twins from different fathers. In theory, IVF could lead to a multiple pregnancy, even if a woman isn’t trying to get pregnant with twins!
In some cases, a woman might already have a fertilized egg or embryo implanted in her uterus when she spontaneously ovulates again. If she has sex while the second egg is viable, the phenomenon known as superfetation can occur. The resulting twins might have the same dad, or they could be twins with different fathers.
But, how do these processes really work?
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Heteropaternal superfecundation occurs when twins are conceived from eggs released during the same ovulation. Although rare, a woman can release multiple eggs during a single menstrual cycle. If she only has intercourse with one man, she can become pregnant with fraternal twins with the same father. Genetically, the twins come from different eggs and sperm, but they have the same parents.
If the woman has released multiple eggs and has sex with different men, heteropaternal superfecundation can occur. Remember: the egg is viable for about 24 hours, and sperm can live inside the uterus for up to six days. That means if the woman has sex with two different men within that window, heteropaternal superfecundation can occur.
Although heteropaternal superfetation also results in fraternal twins, it’s an entirely different process. In very rare cases, some women ovulate after becoming pregnant. This second ovulation can happen days, weeks, or even months after the first egg has been fertilized. In some cases, ovulation can happen during the first trimester of pregnancy.
Heteropaternal superfetation results in two embryos in different developmental stages throughout the pregnancy. However, they’re still usually considered twins. Even though one of the babies will be less developed than the other, they can still both be born through the same labor and delivery.
Fun fact: even though superfetation is extremely rare in humans, it’s quite common in other species like rabbits and fish. There are many factors that make superfetation so uncommon in humans. One is the formation of a mucus plug at the entrance of the cervix after a woman becomes pregnant.
This mucus plug makes it much more difficult for sperm to travel up the cervix and fertilize a new egg. Even if the egg does become fertilized, having another developing embryo in the womb could mean that there isn’t enough space for a new implantation.
Having twins from different fathers is an extremely rare event. Since twins that result from heteropaternal superfecundation develop at the same rate, their paternity can go unnoticed in many cases.
One study found that twins with different fathers occurred in 2.4% of all fraternal twins whose parents had been involved in a paternity lawsuit. Even though 2.4% might seem like a high percentage, this study only researched a small number of pregnancies.
Most reported cases of superfecundation and superfetation have occurred in women undergoing fertility treatments. These treatments make the release of multiple eggs much more likely.
Only 10 cases of superfetation have ever been reported. Research has shown that superfetation might actually occur in as many as 0.3% of all pregnancies. However, many women lose one of the pregnancies early on, and so the real statistics remain unknown.
If you’re pregnant with twins and you had sex with different men around the same time, having twins from different dads is a remote possibility. Although bipaternal twins have resulted from IVF treatments gone wrong, this is also extremely unlikely.
A physician can confirm superfetation if an ultrasound shows that you’re carrying twins at different developmental stages, which is called a growth discordance. The younger baby could end up being born prematurely, but both can be healthy.
Both heteropaternal superfecundation and superfetation are ultimately diagnosed through genetic testing to determine the babies’ biological fathers. This could be done in addition to your routine prenatal testing or after the babies are born. If you have twins who look different, don’t automatically assume that they have different fathers. It’s entirely possible for twins to have the same dad and look completely different — that’s the fun of genetics!
Although heteropaternal superfecundation and heteropaternal superfetation are interesting phenomena, they’re also highly uncommon. Even if you’ve had sex with different men, it’s far more common to have fraternal or even identical twins by the same father.
If you’re having twins, there are many other things to think about, from decorating their nursery to choosing the right pregnancy bra. At the end of the day, having twins from different fathers, although scientifically possible, is very unlikely to happen to you.
McNamara HC et al., A review of the mechanisms and evidence for typical and atypical twinning, Am J Obst Gynecol., 2016 Feb; 214(2):172-191