The presence of twins within a family lineage can suggest a biological predisposition, particularly for fraternal twins. This article explores the scientific factors that influence the likelihood of fraternal twins appearing across generations.
What Are Fraternal Twins?
Fraternal twins, also known as dizygotic twins, form when two separate eggs are fertilized by two different sperm cells during the same menstrual cycle. Unlike identical twins, fraternal twins are genetically distinct. They share approximately 50% of their DNA, making them no more genetically similar than any other siblings born at different times.
The Genetic Link
Fraternal twins often run in families due to a genetic predisposition in the mother to hyperovulate. Hyperovulation means the ovaries release more than one egg during a single menstrual cycle, increasing the chance that multiple eggs can be fertilized. This genetic trait is passed down through the maternal line, meaning a woman is more likely to conceive fraternal twins if her mother or sister has had them. While a father can carry the gene for hyperovulation and pass it to his daughters, he will not be more likely to have twins himself.
Research identifies specific gene variants influencing this increased ovulation rate. Two variants, near the FSHB and SMAD3 genes, link to a higher probability of fraternal twinning. The FSHB gene variant is associated with higher levels of follicle-stimulating hormone (FSH), which can lead to multiple egg releases. The SMAD3 gene variant affects how responsive the ovaries are to FSH. If a woman possesses both variants, her relative odds of having fraternal twins can increase by approximately 29%.
Other Influencing Factors
Beyond genetics, several non-genetic factors can also increase the likelihood of conceiving fraternal twins. Maternal age is a significant factor, with women in their 30s and 40s having a higher chance of conceiving fraternal twins. This is partly because older women may have higher levels of follicle-stimulating hormone (FSH), which can lead to the release of more than one egg. The number of previous pregnancies can also play a role, with women who have had more pregnancies experiencing increased odds of twins.
Additionally, ethnicity influences twinning rates; the Yoruba people in Nigeria have one of the highest rates of dizygotic twinning globally, while rates are lower in South and Southeast Asia. Fertility treatments, such as in vitro fertilization (IVF) and ovarian stimulation, substantially increase the chance of multiple births because they often involve the transfer of multiple embryos or the stimulation of multiple egg releases. Higher body mass index (BMI) and taller height have also been associated with an increased likelihood of fraternal twins.