You may have heard about superfecundation and might be wondering a few things about it. Can a baby have two different fathers? Can a woman get pregnant if she’s already pregnant? Can the same sperm fertilize two eggs or can two different sperm fertilize one egg? Let’s have a look at what superfecundation is while we answer all these questions.
What is superfecundation?
Superfecundation is when a woman gets pregnant more than once in the same menstrual cycle, from different sexual encounters. This will be a twin pregnancy, where both twins have been conceived within hours or days from each other.
In this case, there are two separate ova and each one of them is fecundated by a different sperm cell. When the sperm cells that fecundate each of these different eggs come from different fathers we call this phenomenon superfecundation.
An overview of the menstrual cycle
On average, a woman’s menstrual cycle lasts 28 days. On day one, the first day of her period, various hormones are secreted; they modify the inner lining of the womb and prepare it to nurture the fertilized egg.
Around the middle of the cycle, the woman releases an egg. This moment is called ovulation and the woman is usually fertile for 2 to 5 days. If the woman has sexual intercourse and the egg is fertilized, hormonal changes happen and the pregnancy process starts to develop. If a second, or third, egg is released, this will probably happen around the same time as, or before, the release of the fertilized egg, because once fertilization occurs the hormones that start to work on this pregnancy will most likely stop the release of a new egg.
You can read more about the menstrual cycle and its hormonal changes in Menstrual Cycle – Stages, Phases And What To Expect.
How does superfecundation happen?
A woman’s fertile window starts one or two days before ovulation happens and finishes a couple of days after. Although it’s unusual, she sometimes releases more than one egg. If she has sex with different men, then different sperm cells from different men might each fertilize an egg and heteropaternal (different father) superfecundation might occur.
These babies will be half siblings. They have the same mother and share their mother’s uterus at the same time in the same pregnancy, as fraternal twins, but they have different fathers. This would be known as a dual paternity of two twins. It’s a very atypical twinning – a rare event where twin siblings will have different fathers.
Superfetation and superfecundation
These are two different phenomena; both of them are much more common in other animals.
In superfecundation, both pregnancies start within hours of each other. A woman has had sex with two different men in the same cycle.
In superfetation, a new pregnancy occurs when the mother is already pregnant from a previous cycle.
Superfecundation and superfetation have been described by Hippocrates and Aristotle.
Gould and Pyle did an extensive review of superfetation, published over a century ago when paternity testing was still far from being developed. Many cases were studied where twins were born with very different birthweights and what looked like different gestational ages. Every reported twin pregnancy ending with two babies with noticeable different gestational ages was studied and important results were observed.
How common is superfecundation?
Heteropaternal superfecundation is quite common in mammals like cats, dogs and cows but not very common in human beings. This heteropaternal superfecundation case report from the Australian Journal of Forensic Sciences states how rare superfecundation is in human beings. The research also claims there are probably many more cases than those reported, as the absence of confirmation is quite common. A paternity test isn’t performed unless there’s a need to determine paternity, so many superfecundation cases might go unnoticed.
Can you get pregnant while pregnant?
Although this is quite rare it can happen and it’s called superfetation. Two babies are conceived from separate acts in two different cycles. These babies can be from the same father or two different men.
When heteropaternal superfecundation occurs, the babies are from different fathers. These babies will be fraternal twins and half-siblings.
Fraternal twins and identical twins
Fraternal twins are also called dizygotic twins. This means that the babies come from two completely different eggs, each one fertilized by a different sperm. If the sperm cells that fertilize both eggs are from the same man these babies will share the same amount of DNA as siblings. If the eggs are fertilized by two different sperm cells from two different men heteropaternal superfecundation occurs.
Identical twins come from the normal fertilization of one egg by one sperm cell. During the first few hours following conception, this fertilized egg splits into two and each one of the new identical embryos carries on developing on its own. Identical twins will have the same genetic information.
As you can see, heteropaternal superfecundation can only happen with fraternal twins and never with identical twins.
There could also be the case of triplets, where two babies are identical twins and the third baby is a fraternal twin. If superfecundation has occurred the third baby will be a half-brother or sister to his or her two identical twin siblings.
Can it happen?
Yes it can but, as you might have guessed, the likelihood of it happening is very, very rare.
Can a baby have two biological fathers?
No, that is not possible. The baby’s mother might have had many sexual encounters with different men and she might not know who the father of the baby is. However, the baby cannot have different biological fathers. The baby will only have one biological father: the owner of the sperm that fecundated the egg that became the baby.
Unless there are very obvious external identifiable characteristics (men of different races) the only way to know who the baby’s father is by a DNA test. Paternity testing is an easy and definite way to determine who the biological father of the baby really is.
Can two sperm fertilize the same egg?
No, that is not possible. When the sperm fertilizes the egg, a protective barrier forms around the egg to stop any other sperm from fertilizing it. In the unlikely event that two sperms fertilize the same egg, the egg will not develop, as the amount of genetic information will be wrong and the process will immediately stop. The woman would miscarry the embryo without even knowing that the pregnancy had taken place.
Can triplets have three different fathers?
Yes, it is possible. Because superfecundation happens from different sexual encounters, it’s possible that a woman is pregnant with triplets or twins with different fathers. The number of babies depends on the number of ova released by the woman in that one cycle. If she has sex with different men during her ovulation it’s possible, although quite unusual, for the sperm that fertilizes each egg to come from different men. Those babies would be fraternal twins with different fathers.