Twins are classified into two main types: identical (monozygotic) and fraternal (dizygotic). Identical twins result from a single fertilized egg that splits early in development, making them genetic copies of one another; this process is considered a random event with no known hereditary link. Fraternal twins develop from two separate eggs fertilized by two distinct sperm cells, making them genetically no more alike than any other siblings born at different times. The predisposition for fraternal twinning is hereditary, as it is tied to a woman’s biological tendency to release multiple eggs during a single menstrual cycle.
The Biological Mechanism of Fraternal Twinning
The formation of fraternal twins is rooted in hyperovulation, which is the release of more than one egg during a single menstrual cycle. In a typical cycle, a woman’s ovaries release only one dominant egg. When hyperovulation occurs, usually two eggs—but sometimes more—are simultaneously released. Each separate egg must then be fertilized by a separate sperm cell, resulting in two distinct zygotes.
These two fertilized eggs then implant independently into the uterine wall and develop as separate embryos, each with its own placenta and amniotic sac. Because they originate from two separate fertilization events, fraternal twins share approximately 50% of their DNA. This is a key difference from identical twins, which share nearly 100% of their DNA. Fraternal twins can be of the same or opposite sexes, and they may look quite different from one another.
The Inherited Trait and Genetic Link
The hereditary component of fraternal twinning is directly linked to genetic factors that influence a woman’s likelihood of hyperovulation. Research has identified specific gene variants that contribute to this maternal trait by affecting the regulation of reproductive hormones. One gene variant, located near the Follicle Stimulating Hormone Beta Polypeptide (\(FSHB\)) gene, is associated with naturally higher levels of Follicle-Stimulating Hormone (FSH). Elevated FSH levels can cause multiple follicles to mature and release eggs simultaneously.
Another gene variant is found in the \(SMAD3\) gene, which appears to influence how sensitive the ovaries are to FSH. Even with normal FSH levels, a woman may still release multiple eggs if her ovaries are hyper-responsive due to this variant. If a woman carries both of these specific gene variants, her chance of having fraternal twins is increased by nearly 29%.
The inheritance pattern of this trait is strictly maternal, meaning the genes must be passed down through the female line to be expressed. A woman’s father can carry and pass the genetic variants for hyperovulation to his daughter, but he cannot express the trait himself since he does not ovulate. Therefore, a woman is more likely to have fraternal twins if her mother, sister, or other close female relatives have had them. The increased probability is estimated to be about two times higher than the general population if a close relative has had dizygotic twins.
Non-Genetic Factors Influencing Twinning Rates
While genetics predispose a woman to hyperovulation, several non-inherited factors also influence the rate of fraternal twinning. Maternal age is one of the most prominent factors, as the rate of dizygotic twins increases steadily up to around age 37 before declining. This is believed to be due to the body producing higher levels of FSH as a woman approaches menopause, which can inadvertently lead to the release of multiple eggs.
Another factor is parity, which refers to the number of previous pregnancies a woman has had, with twinning rates increasing with each successive birth. Racial and ethnic background also show variation in fraternal twinning rates. The highest natural rates are observed in certain African populations, and the lowest rates are found in Asian populations.
The most significant non-genetic influence today is the use of Assisted Reproductive Technology (ART) and ovulation-stimulating drugs. Fertility treatments, such as in vitro fertilization (IVF), often involve transferring multiple embryos into the uterus, which directly results in a higher rate of fraternal twins. Furthermore, medications designed to induce ovulation can intentionally cause hyperovulation, leading to the release and subsequent fertilization of multiple eggs.