Is it Possible for Twins to Have Different Fathers?
It is known that two siblings can have different fathers, which makes them half-siblings. However, is this possible in twins? There are two main types of twins: identical and fraternal. Identical twins occur when a single egg is fertilized by a single sperm and forms a zygote that then divides into two separate embrios. On the other hand, fraternal twins occur when a woman releases more than one egg during her ovulation period, which allows for different sperm to fertilize them. When a woman releases more than one egg there is the possibility that twins come from different fathers. However, the odds of this happening are low. In this OneHowTo article we're going to explain the possibility for twins to have different fathers.
Different types of twins
Before talking about the possibility of twins having different fathers it is important to know the differences between identical (monozygotic) twins and fraternal (dizygotic) twins. Identical twins come from a single egg that divides in two, for which these twins share the same exact DNA. For this reason, this type of twins will always have the same sex. On the contrary, fraternal twins occur when two or more eggs are fertilzed by different sperms. In this case, the twins will look alike because they are siblings, but they won't be identical. Also, they can have different sexes.
Having said that, it will be impossible that identical twins have different fathers, as they started first as a single egg that later divided. However, fraternal twins come from different eggs and different sperms, so it could be possible that they came from different fathers as well.
Possibility of becoming pregnant of two different men
As strange as it might sound, yes, it is possible for twins to have different fathers. This is possible when two eggs from the same mother are fertilized by two different fathers within the same ovulation period.
Normally, women only ovulate one single egg per cycle. However, some women are more prone to release more than one egg during their cycle, which makes fraternal (dizygotic) twins possible. When we see twins we might think that both their eggs were fertilized during the same act of intercourse. However, this doesn't always have to be this way. A woman usually has a 24 hour fertility window, which means that if she releases more than one egg and has intercourse without protection more than once during her fertile day, she can become multiply pregnant. When this happens it is called superfecundation: two eggs released during the woman's fertile days fertilized during different acts of intercourse. For instance, let's suppose that a woman has ovulated two eggs and has sex in the morning and in the evening during the same day. There is the possibility that the two eggs are fertilized at two different times.
If this happens with two different men it is called heteropaternal superfecundation.
How common is heteropaternal superfecundation?
Although heteropaternal superfecundation is possible, there is no clear evidence that it is common among twin pregnancies. However, one study from the journal Acta Genet Med Gemellol suggested that one in 400 fraternal twin pregnancies in the US is bipaternal.
For heteropaternal superfecundation to occur some things need to happen:
- The mother must release two eggs during ovulation.
- The two eggs need to be fertilized by two sperms. Sometimes a woman does release two eggs during the cycle but only one is fertilized. The odds of one egg being fertilized are small, and the odds of two eggs fertilized are even smaller.
- The woman has to have intercourse with two different men over a very short period of time. The ovulated eggs are only available during a short period of time (around 24 hours) and the sperm needs to be in the fallopian tube during that time for fertilisation to happen.
- Sperm of both men need to reach the eggs and fertilize them.
The odds of having a heteropaternal superfecundation are very small and there have only been seven cases reported worldwide since 2011. However, it is also true that these pregnancies might occur more regularly but go unnoticed.
Heteropaternal superfecundation in other mammals
Heteropaternal superfecundation is not very common in humans, but it is among other mammals, mainly in cats and dogs. Female dogs and cats ovulate multiple eggs each time, making superfecundation easier. This happens mostly in stray dogs and cats, which can have babies from the same litter that look completely different.
This article is merely informative, oneHOWTO does not have the authority to prescribe any medical treatments or create a diagnosis. We invite you to visit your doctor if you have any type of condition or pain.
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- If two twins don't look alike don't immediately asume they come from different fathers. As siblings, it is possible that fraternal twins don't look similar as they come from different eggs and sperm and have a different DNA.