Having twins is possible even without a family history of multiple births. The occurrence of twins involves two distinct biological events influenced by different factors. While one type of twinning can be inherited, the other is a spontaneous occurrence that happens randomly. The likelihood of twins is influenced by random chance, genetic predisposition, and external medical factors.
Understanding Identical Versus Fraternal Twins
Identical twins, or monozygotic twins, form when a single fertilized egg splits into two separate embryos early in development. This splitting process is random and occurs at a consistent rate of about three to four births per 1,000 worldwide, regardless of family history.
Fraternal twins, or dizygotic twins, result from the fertilization of two distinct eggs by two different sperm during the same cycle. These twins are genetically no more alike than any other siblings. Their formation requires the mother’s body to release more than one egg in a single menstrual cycle, a process called hyperovulation.
How Genetics Influences Twinning
The family history connection applies almost exclusively to fraternal twins. The genetic component is tied to the gene that predisposes a woman to hyperovulation. This tendency to release multiple eggs increases the chance of having dizygotic twins naturally.
This hyperovulation gene can be inherited from either parent. However, it only directly affects the probability of a twin pregnancy if the carrier is the person who ovulates. A woman who inherits this trait is statistically more likely to conceive fraternal twins. A man may carry the gene and pass it to his daughter, but it will not increase his own chance of having twins with a partner who does not have the gene.
If there is no family history of fraternal twins, there is a lower natural probability of hyperovulation. However, the chance of conceiving identical twins remains the same for everyone. Identical twinning is a spontaneous cellular event unrelated to genetics. This distinction explains why a lack of family history does not eliminate the possibility of a twin pregnancy.
Non-Inherited Factors That Increase the Likelihood
Many factors unrelated to genetics can significantly increase the likelihood of conceiving fraternal twins, often overriding a lack of family history. The most influential factor is the use of Assisted Reproductive Technology (ART), such as IVF and ovulation-inducing medications. Fertility drugs stimulate the ovaries, which can cause hyperovulation and result in the release of multiple eggs.
In IVF, transferring multiple embryos into the uterus directly raises the odds of a twin birth. The overall rise in the twin birth rate over the past few decades is largely attributed to the increasing use of these fertility treatments. These medical interventions create the necessary conditions for fraternal twinning, even without the hyperovulation gene.
A woman’s age also plays a role, as the likelihood of fraternal twins increases with advancing maternal age, particularly after age 35. As a woman nears menopause, levels of Follicle-Stimulating Hormone (FSH) naturally increase, which can stimulate the ovaries to release more than one egg. Having had previous pregnancies, known as parity, slightly increases the chance of conceiving twins. Other physiological factors, such as higher body mass index (BMI) or greater maternal height, have also been associated with a slightly elevated chance of hyperovulation.