Can twins have different fathers?

Yes they can. It sometimes happens in the lab when two different men's sperm are added to a dish with a woman's eggs. But it can happen in nature too.^1,2

There it happens when a woman releases two eggs in a cycle. This is the first step in making any fraternal twins.

The difference in this case is that each egg is fertilized by a different man's sperm. This can happen if the woman is with two different men at a time when she can become pregnant.

The process goes by the awful name heteropaternal superfecundation. I promise not to use those words ever again in this answer!

This sort of thing is pretty common in the animal world but no one seems to have a good number on how common it is in people.³ A number floating around the internet is that 1 in 12 sets of fraternal twins worldwide have different dads. Another number floating around is 1 in 13,000. So there is quite a range of possibilities here!

The first number is supposedly associated with the National Institutes of Health which makes it sound reliable. I have looked and looked and have not been able to find the original study (and frankly it seems shockingly high). As for the second number, I can't find its reference anywhere.

A study from 1992 suggested that around 2.4% of fraternal twins involved in a paternity test had different dads.³ And a friend of mine at a paternity testing company told me that her company sees about two of these cases a year. But since her company doesn't track how many fraternal twins they check, we can't really get a percentage from that.

So all we can say at this point is that it can happen (as recent stories from Turkey and Texas show). What we can't say is how common it is.

Two Eggs, Two Dads

A woman has to release two eggs in a single cycle to have a chance at having fraternal twins. This can happen with any woman although it is more common in some. Around 8 in every 1000 live births is a set of fraternal twins!⁴

Once two eggs have been released, both need to be fertilized and implant in the uterus. Assuming everything goes well, after 7-9 months, twins are born.

Obviously two different men need to be involved in order for each twin to have a different dad. The dads have to both contribute within five days of each other at the right time in a woman's cycle. This is possible because sperm can survive that long inside of a woman's body.

Even less common than this route to twins with different dads is something called superfetation. In this case, an egg is released and fertilized after a woman is already pregnant.¹ Sometimes these twins can have different dads too.

To make things even more complicated, twins sometimes fuse at a very early stage and grow into a single person. This person is called a chimera.⁵

Chimeras are people who have two sets of cells. One set comes from one fertilized egg and the other from a different fertilized egg. If each egg was fertilized by a different man, then the chimera would have two sets of cells from half siblings.

No one knows how common chimeras are either but again, we know they're out there because there have been reported cases.⁴ However, any estimates about the frequency of chimeras are probably undercounts. Most people don’t get tested for chimerism, so they would never know!

And Genetics is Involved How?

This is all very interesting but doesn't seem to have a lot to do with genetics. In fact, I almost didn't post the answer because of this.

But these sorts of things are important when thinking about paternity tests. For example, if you are having testing done with fraternal twins, it may be worth the extra money to test both of them. Because their DNA may show they have different dads!

And chimeras complicate things even further. It is hard enough to know what to do with chimeras from full siblings. Half siblings are even more complex.

The combination of two different dads plus a fusion is incredibly rare. So rare that people probably shouldn't worry about it too much. Unless further studies show that chimeras are more common than we anyone thought they were.