The birth rate of identical (monozygotic or MZ) twins is the same all over the world. It is approximately four per thousand births. But the birth rate of fraternal (dizygotic or DZ) twins varies. DZ twins occur when two eggs are released in the same menstrual cycle and are fertilized by two different sperm. The factors discussed below can affect the process of DZ twinning.
Genetics: There appear to be genetic factors that can cause hyperovulation, which is the release of multiple eggs during ovulation. These genetic factors have not been identified. There is no genetic testing available for them.
Maternal age: The older a woman is, the greater her chance of having DZ twins. This may be due to the higher level of follicle-stimulating hormone (FSH) that older women have as they approach menopause. FSH stimulates ovulation, so an increased level of FSH may cause multiple eggs to be released. Older women are statistically less likely to get pregnant. But if they do get pregnant, they are more likely to have DZ twins!
Fertility treatments: The use of fertility treatments, such as fertility drugs and in vitro fertilization (IVF), often causes DZ twinning. Fertility drugs given to women to stimulate ovulation can cause the release of more than one egg at a time. DZ twins may occur if two eggs are fertilized. In IVF, eggs are surgically removed from a woman’s ovaries and combined with sperm in a laboratory. The resulting embryos are transferred into the woman’s womb to develop. To increase the chances of a successful pregnancy, more than one embryo may be transferred into her womb. This may result in the birth of DZ twins.
Race: The occurrence of DZ twins varies by race. DZ twins are more common than average in African Americans populations, and less common in Hispanic and Asian populations.
Nutrition: Being well nourished increases the chance of DZ twinning. The rate of DZ twinning decreases with malnutrition. Some dietary habits, such as eating certain yams grown in Africa, can increase the chances of DZ twinning.
Number of pregnancies: The more pregnancies a woman has had, the greater are her chances of having DZ twins.
Time of year: The most DZ twins are conceived in July. The fewest DZ twins are conceived in January. This may be due to the effect of the length of daylight on the secretion of FSH.
Height: Taller women are more likely to have twins than smaller women. This may be because taller people have more of a substance called insulin-like growth factor (IGF). IGF has been linked to both greater height and increased ovulation.