Twins occur when two babies are born to one mother after a single pregnancy. If they're identical (monozygotic), they formed when one zygote, created with just one egg and one sperm, splits into two new eggs, resulting in two embryos rather than one. Each embryo then develops into a separate fetus.

Fraternal (dizygotic) twins, meanwhile, occur when two separate eggs are fertilized by two separate sperm.﻿﻿ Fraternal twins share only 50% of the same genes, whereas identical twins share 100%, which explains why fraternal twins usually don't look nearly as alike as identical ones do.

Unless a set of twins' birth mother served as a surrogate or used donor eggs, there would be no question that she's the mother of both twins. But you may be left wondering: Can twins have different fathers? The answer is yes, but only in cases in which they're fraternal, as identical twins form from a single egg/sperm combination and thus cannot have different fathers.

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Twins With Different Fathers

As technology has improved the accuracy and accessibility of genetic testing, people have been able to receive genetic testing to determine when fraternal twins have two different fathers. Fraternal twins are the result of hyperovulation, the release of multiple eggs in a single cycle. Superfecundation describes a situation in which each of the eggs are fertilized by sperm from different men, leading them to have different biological fathers (making the twins half-siblings). The appropriate term to describe this situation is heteropaternal superfecundation.﻿﻿

There is little research on the prevalence of heteropaternal superfecundation in the general population, but some researchers found that in a parentage test database of 39,000 records, 2.4% of dizygotic twins whose parents were involved in paternity suits were a result of heteropaternal superfecundation.﻿﻿

Some researchers believe that the frequency of heteropaternal superfecundation, which is widely assumed to be very rare, is underestimated in the general population.﻿﻿

Examples of Heteropaternal Superfecundation

One way heteropaternal superfecundation could occur is if a woman has sexual intercourse with two different men within the same timeframe that both embryos are conceived. For example, in New Jersey, a mother of twins underwent paternity testing when applying for public assistance. After the test showed that her partner was only the father of one of her twins, she admitted that she had had sex with another man within the same week that her twins were conceived.

Similarly, a mother of twins in Texas acknowledged that she was having an affair with another man when her twins were conceived.﻿﻿ Paternity testing revealed that her fiancee was indeed the father of one of the twin boys, but that another man was the biological father of the other twin.

While we might assume all instances of twins with different fathers are the result of the mother having sexual intercourse with different partners, this isn't the only scenario in which it could happen. This situation can also occur when twins are the result of fertility treatments. For example, in a mixup at a lab, equipment had been used twice, causing another man’s sperm to be mixed with that of the intended father resulting in Dutch twin boys conceived through in vitro fertilization (IVF) with different fathers.﻿﻿

How Paternity Testing Is Performed

Paternity testing can be performed either before or after twins are born. Post-natal paternity testing can be done by collecting and testing blood from the baby's umbilical cord at birth (a blood test performed at a lab) or a buccal (cheek) swab taken either at a lab or at home and then later sent to a lab. The presumed mother and father's DNA is usually obtained this way as well.

Prenatal paternity testing is a bit more complicated, however, and can be more invasive depending on which method is performed.

Non-Invasive Prenatal Paternity (NIPP)

This test involves collecting blood from both the mother and the presumed father, which is then tested after preserving and analyzing the baby's DNA found in the mother's bloodstream. It can be performed any time after eight weeks of pregnancy and is 99.9% accurate.﻿﻿

Amniocentesis

Amniocentesis involves a doctor using ultrasound technology while inserting a thin needle into your uterus through your abdomen to draw amniotic fluid for genetic testing (most often used for prenatal diagnosis of chromosomal abnormalities and other medical conditions in the fetus). This test can be performed in the second trimester and carries a small risk of harm to the baby and causing miscarriage.

Chorionic Villus Sampling (CVS)

Chorionic villus sampling (CVS) is also often used to check for chromosomal disorders. It involves a doctor inserting a thin needle or tube guided by ultrasound from the vagina and through the cervix to obtain chorionic villi, or small pieces of tissue attached to the uterine wall. The fetus and this tissue come from the same fertilized egg and have the same genetic makeup, thus allowing it to be used for paternity testing against the alleged parents' DNA. CVS can be performed earlier in a pregnancy, from weeks 10 to 13. As with amniocentesis, CVS poses a small risk of miscarriage, as well as cramping, bleeding, and uterine infection.﻿﻿

A Word From Verywell

Because all forms of paternity testing will involve collecting DNA from both parents, there may be an awkward conversation at some point in your pregnancy or after the birth of your twins if there is any doubt over paternity. By researching your options and preparing yourself for the procedures and results, you can make the process a bit less stressful. As always, talk to your doctor about any concerns regarding your pregnancy and any associated procedures.