Can you have twins from different fathers?

Almost 2 in every 100 births in Ireland result in twins.

In this article, we explore the science behind a twin phenomenon known as heteropaternal superfecundation.  This is when a pair of twins from the same birth have different fathers due to two instances of fertilisation within the same menstrual cycle.

An AlphaBiolabs paternity test can help families find out whether their twins were affected by heteropaternal superfecundation and therefore have different biological fathers. This test can be performed during pregnancy or after birth.

What are twins?

Twins are born when one pregnancy produces two offspring. Twins can be classified into two main types: monozygotic (identical) and fraternal (dizygotic or non-identical). The two types of twins are different in the way they originate, therefore resulting in differences in the amounts of DNA that they share.

Identical (monozygotic) twins

These twins originate from the same fertilised egg (zygote) and share almost 100% of their DNA. Shortly after conception, the developing embryo splits into two genetically identical embryos. Once split, the two embryos begin to develop alongside each other and form two identical foetuses.

Identical twins are usually the same sex, and share almost all genetic material, with any differences being a result of random mutations that occurred after the embryonic split. However, these differences are generally minimal, meaning that the twins are usually very similar in appearance and can be difficult to tell apart.

Fraternal (non-identical/dizygotic) twins

These twins originate from the fertilisation of two separate eggs by two separate sperm cells. Therefore, fraternal twins only share around 50% of their DNA, and have the same genetic relatedness as siblings from separate births. Fraternal twins can be the same sex, or different sexes and may or may not closely resemble one another – just like any other siblings.

Can you have twins from different fathers?

It is possible for a woman to have twins, with each twin having a different father. This phenomenon is known as heteropaternal superfecundation.

This can occur when a woman releases multiple eggs during her menstural cycle and engages in sexual intercourse with different partners in close succession.

For this to happen, a woman needs to release an egg that is fertilised by one man, then release a second egg that is fertilised by another man. Both fertilisation events usually occur within 12-24 hours, which is known as the ovulation period. However, sperm can survive within the uterus for up to several days so, although highly unlikely, it is possible for heteropaternal superfecundation to occur from sexual encounters that are several days apart.

How common is it?

It is rare for twins to have different fathers.

Heteropaternal superfecundation is considered extremely rare in humans. However, its exact prevalence is difficult to determine, due to the lack of thorough genetic testing in twins.

The likelihood of heteropaternal superfecundation occurring is influenced by various factors. Factors include the probability of multiple ovulation (increased in women with hyper-ovulation) and engaging in sexual activity with different partners around the same time.

How can I find out whether my twins have different fathers?

If you suspect that your twins may have different fathers and wish to confirm this through DNA testing, you can consider taking a paternity test. This can be performed during pregnancy by Non-Invasive Prenatal Paternity Testing (NIPPT), or any time after birth through a paternity test.

Paternity testing after the twins are born

DNA paternity testing is the most accurate way to determine whether your twins have different fathers. DNA testing requires a simple cheek swab and can be done after birth, or later in life. By comparing the twins’ DNA with the DNA from the potential father(s), the paternity of each twin can be determined.

Additionally, a zygosity twin test can be carried out to determine whether the twins are identical or fraternal (non-identical). This test compares the genetic profile of the twins and determines whether they share 100% of their DNA, indicating that they are identical, or 50% – 70% of their DNA, meaning that they are fraternal twins.

Paternity testing before the twins are born

A prenatal paternity test is a DNA test that determines the biological relationship between an alleged father and an unborn child. Unlike traditional paternity tests conducted after a child is born, prenatal paternity testing can be performed during pregnancy.

A Non-Invasive Prenatal Paternity Test (NIPPT) can achieve this by using a simple blood sample drawn from the mother’s arm, from as early as 7 weeks into pregnancy. NIPPT works by analysing the baby’s DNA that circulates within the pregnant mother’s bloodstream. This DNA can then be compared with the DNA from the potential fathers to determine paternity.

Traditional prenatal paternity tests such as amniocentesis or chorionic villus sampling (CVS) are invasive and therefore pose an increased risk of miscarriage.  By contrast, NIPPT is a simple blood test and is therefore completely risk-free for both the mother and the baby.