Twins, the simultaneous birth of two offspring from the same pregnancy, often spark curiosity about their origins, especially whether they are inherited. While the idea of twins “running in the family” is a common belief, the genetic truth behind it is more nuanced and depends heavily on the type of twins.
Understanding Twin Types
Twins are categorized into two primary types: monozygotic, commonly known as identical twins, and dizygotic, referred to as fraternal twins. Monozygotic twins originate from a single fertilized egg that splits into two separate embryos early in development. Because they develop from one zygote, identical twins share nearly identical genetic information and are always of the same sex. The timing of this split can influence whether they share a placenta and amniotic sac.
In contrast, dizygotic or fraternal twins result from two separate eggs, each fertilized by a different sperm cell during the same menstrual cycle. These twins are no more genetically alike than any other siblings born at different times, sharing about 50% of their genes. Fraternal twins can be of the same or different sexes.
How Maternal Genes Influence Twinning
The genetic influence on twinning predominantly applies to fraternal twins, stemming from the mother’s side of the family. This genetic link is associated with a woman’s predisposition to hyperovulation, which is the release of more than one egg during a single menstrual cycle. Two gene variants identified in 2016 are thought to increase the odds of fraternal twins by affecting follicle-stimulating hormone (FSH) production or how ovaries respond to it.
A father can carry and pass on these hyperovulation genes to his daughters. These genes will only increase the chance of twinning in his daughters, as men do not ovulate. A man’s family history of twins does not increase his own likelihood of having twins, but it can increase his daughter’s chances. Conversely, identical twinning is generally considered a random event, not hereditary.
Other Factors in Twin Births
Beyond maternal genetics, several other factors can influence the likelihood of twin births. Maternal age is a factor; women in their 30s and 40s have higher levels of sex hormones, which can stimulate the ovaries to release multiple eggs, increasing the probability of twin pregnancy.
Ethnicity also plays a role, with some groups exhibiting higher rates of dizygotic twinning. For example, the Yoruba people of West Africa have the highest rate globally, while Asian populations have lower rates. The number of previous pregnancies can also increase the odds of conceiving twins, with each successive pregnancy slightly elevating the chances. Fertility treatments, such as in vitro fertilization (IVF) and ovulation-stimulating drugs, significantly increase the likelihood of twin births by prompting the release of multiple eggs or transferring multiple embryos.