Two kinds of twins, two different stories
Identical twins (monozygotic) come from a single fertilised egg that splits into two genetically identical embryos. It happens by chance during early development, and its rate is remarkably stable worldwide — around 3-4 per 1,000 births — with no demonstrated link to maternal age, family genetics or ethnicity.
Fraternal twins (dizygotic) come from a double ovulation: two separate eggs fertilised by two separate sperm in the same cycle. Unlike identical twins, this double ovulation IS influenced by identifiable factors — the ones covered in the rest of this page.
Maternal age
The odds of spontaneous double ovulation rise with maternal age, especially from age 35 onward, because of the higher follicle-stimulating hormone (FSH) levels typical of that stage — the same hormonal shift that eventually leads to menopause causes, in some cycles beforehand, the release of more than one egg. Twin rates drop again after menopause, as expected.
Family history (and why only the mother's side counts)
A hereditary predisposition to double ovulation exists, and genetic studies — such as Mbarek and colleagues, published in the American Journal of Human Genetics in 2016 — have traced it to specific variants associated with releasing more than one egg per cycle. Only the mother can pass on that predisposition, because she is the one ovulating: a father having twins in his family does nothing to change his partner's odds of releasing two eggs.
It is one of the most widespread myths on this topic, and the genetics explains it clearly: the family history that matters is the mother's and her maternal line (mother, sisters, maternal grandmother), not the father's.
Fertility treatments
By far the factor with the largest effect. Treatments that stimulate ovulation (such as clomiphene citrate or gonadotropins) are specifically aimed at producing several eggs per cycle, and IVF historically transferred more than one embryo to improve pregnancy odds — a practice that has been shifting toward single-embryo transfer precisely to reduce multiple pregnancies. Fertility-treatment surveillance registries document multiple-pregnancy rates well above those of spontaneous conception, though the exact figure depends too much on the protocol and number of embryos transferred to give a single number here.
Other documented factors
- Ethnic and geographic origin: spontaneous twinning rates vary widely by population — from notably high figures in some West African populations to low figures in East Asia — a pattern described since the classic twinning studies of the 1970s-80s and confirmed since.
- Number of previous pregnancies: having had several prior pregnancies is associated with a slightly higher chance of twins in later pregnancies.
- Maternal height and weight: some studies find a mild association between greater maternal height or BMI and higher odds of double ovulation, though the effect is much smaller than that of age or assisted fertility.