Twins are fascinating and are the subject of many old-wives-tales. If you have a set of twins in your family, a relative may have told you that you have a higher chance of having twins — but do twins really run in families?
As it turns out, some types of twins are hereditary. Suppose you're a gestational parent and a close relative (like a parent or sibling) has fraternal (non-identical) twins. In that case, you're twice as likely to have twins yourself.
Read on to learn what you need to know about the heredity of having twins.
What Causes Twins?
There are two basic types of twins — fraternal (or dizygotic) and identical (or monozygotic) twins.
Fraternal twins come from two separate eggs that two different sperm fertilize. Fraternal twins have separate amniotic sacs and placentas. Fraternal twins are the most common type of twin. Fraternal twins account for 70% of all spontaneous twin pregnancies.
Fraternal twins can have different sexes and appearances and are no more genetically related than any other siblings with the same parents.
Do Fraternal Twins Run in Families?
Two gene variants discovered in 2016 increase the odds of fraternal twins by up to 29%. One of these variants affects the production of follicle-stimulating hormone (FSH), and the other controls how the ovaries respond to FSH. FSH stimulates the ovaries to produce eggs, and these gene variations likely contribute to hyperovulation — the release of more than one egg in a menstrual cycle.
The genetic predisposition for twins only affects gestational parents. A biological, non-gestational parent can pass down these genetic traits to their children. Still, they won't be more likely to raise twins themselves unless the surrogate, gestational parent also has a family history of twins.
Certain genetic factors make fraternal twins run in families. There's a higher probability of having twins if you're the gestational parent and you:
- Are a fraternal twin
- You already have fraternal twins
- A close relative has fraternal twins
Other factors that increase the odds of having fraternal twins include:
- Age: gestational parents in their 30s and 40s are more likely to give birth to twins than younger parents.
- Previous pregnancies: higher numbers of pregnancies increase the odds of twins, especially a prior twin pregnancy.
- Ethnicity: Nigerians have the highest rate of fraternal twins.
Identical twins come from a fertilized egg that splits into two. Depending on how early the egg splits, identical twins may share a single amniotic sac and placenta.
Identical twins have the same DNA. These twins will have the same sex and look very similar in appearance. However, they may not be exactly identical due to environmental factors.
Do Identical Twins Run in Families?
Identical twins are typically not hereditary like fraternal twins and occur in three to four births out of every 1,000 globally. A few families report a higher level of identical twins than expected, so there may be a genetic factor in rare cases. However, the cause of identical twins is typically unknown.
Assisted reproductive technology (ART) may increase the odds of having identical twins. A 2016 study showed that the rate of pregnancies with identical twins was significantly increased in fresh blastocyst transfers during in-vitro fertilization cycles.
A Third Type of Twins?
A third type of twins has been proposed as a theory to explain why some fraternal twins look identical. This third type would be twins resulting from a single egg that split before fertilization and was then fertilized by two separate sperm. This type of twin would share more DNA than fraternal twins but less than identical twins. Medical professionals are still divided on whether or not this occurs.
Do Twins Skip Generations?
Many people believe twins skip a generation, but that's just a myth.
The idea that twins skip generations likely comes from the fact that the genetic factors contributing to twins only come from the gestational parent's side. If a cisgender father's mother had fraternal twins, it's statistically likely that he won't have fraternal twins himself because of the rareness of twins. Still, suppose he passes the genetic tendency for twins down to a child with ovaries. In that case, that child will be more likely to have twins than another family.
Some people who did not have twins themselves but have twin grandchildren may feel that the increased odds of having twins "skipped" a generation in their family. Still, at this time, the idea that twins skip generations is purely anecdotal.