In the United States alone, 114,161 sets of twins were born in 2021. The occurrence of twin births makes many people wonder whether there is a hereditary component to if twins run in families. The answer to this question really just depends.
What Type of Twins Run In Families
There are two types of twins: monozygotic and dizygotic. Monozygotic twins, or identical twins, occur as a result of a splitting of an already fertilized egg while dizygotic twins, or fraternal twins, occur when two individual eggs are fertilized separately. Current research shows that cases of monozygotic twins are random. However, some researchers believe that there could be a genetic factor. They suspect this because of reports of unlikely numbers of monozygotic twin births within single families. So, there is some reason to believe that identical twins run in families. Researchers are much more certain that cases of dizygotic twins can and do run in families.
A Mother’s Influence
Though both mother and fathers can hold certain genetic variations, current research suggests that only the DNA of the mother matters where twin births are concerned. Dizygotic twins are believed to be due to genetic variations that cause hyperovulation. Hyperovulation is the release of more than one egg in a menstrual cycle. As only the mother’s DNA controls the mother’s ovulation, the father’s DNA will have no effect on the chances of a couple having twins. However, a father can pass genetic variations that cause hyperovulation on to his daughter. She may then have a higher chance of twins one day.
Twins Run in Families and So Do Genes
There are several genes that researchers say affect the chances of twin pregnancies. Two of the most well-researched ones are FSHB and SMAD3. Researchers see a link between higher amounts of follicle-stimulating hormone and hyperovulation. A variant of the gene FSHB leads to increased levels of the hormone. This variant may also decrease the likelihood of polycystic ovarian syndrome, a condition that can negatively impact fertility. A variant of SMAD3, on the other hand, could cause twin pregnancies by increasing the cells’ responsiveness to expressions of the FSHB gene. Importantly, there are also other non-genetic factors that determine if twins run in families. These include the mother’s age as well as IVF, or in vitro fertilization.
