The tendency to have twins is often questioned, especially by individuals with a twin sibling or a family history of multiple births. While twin births can seem random, one specific type of twinning is strongly influenced by genetics. Understanding the difference between the two primary types and the genetic predisposition that affects only one reveals the mechanisms that increase the likelihood of conceiving multiples.
The Two Types of Twins
Twins are broadly categorized into two types: monozygotic and dizygotic. Monozygotic twins, commonly referred to as identical twins, result from a single fertilized egg (zygote) that spontaneously splits into two separate embryos early in development. Because they originate from the same egg and sperm, these twins share virtually identical genetic material, making them the same sex and very similar in appearance.
The biological mechanism causing a single zygote to divide is not fully understood and is considered a random, non-hereditary event. The incidence of identical twins remains relatively constant worldwide, occurring in about three to four out of every 1,000 births.
Dizygotic twins, known as fraternal twins, form when two separate eggs are released during a single cycle and are fertilized by two different sperm cells. These twins share approximately 50% of their DNA, making them genetically no more alike than other siblings. Their occurrence is directly tied to specific biological and genetic factors.
Genetic Influence on Twinning
The heritability of twins is exclusively linked to dizygotic (fraternal) twinning. This genetic predisposition centers on hyperovulation—the tendency for a woman’s ovaries to release more than one egg during a single menstrual cycle. The release of multiple eggs significantly increases the chance that more than one egg will be fertilized, resulting in fraternal twins.
This trait is governed by genes that influence a woman’s reproductive hormones, specifically follicle-stimulating hormone (FSH) levels. Research has identified gene variants, such as those in the FSHB and SMAD3 genes, associated with increased hyperovulation. This genetic tendency must be present in the mother, as she is the individual who ovulates.
If a woman has a close relative, such as a mother or sister, who has had fraternal twins, she is statistically more likely to also have them. This genetic mechanism ensures that fraternal twins are not random within certain family lines. This is the only type of twinning subject to inheritance.
Likelihood Based on Parental Role
Whether a twin can have twins depends entirely on the type of twin and the parent’s sex. If a woman is a fraternal twin or has inherited the hyperovulation gene, her chance of conceiving fraternal twins is notably increased. Women who are fraternal twins have a rate of about one set of twins per 60 births, significantly higher than the general population rate of approximately one in 250 natural pregnancies.
The father’s role is different, as he does not ovulate. If a man is a fraternal twin, his chance of fathering twins with his partner is not increased, remaining close to the population average of about one set per 125 births. He cannot directly cause his partner to release multiple eggs, which is the mechanism required for fraternal twinning.
A father who is a fraternal twin can carry the hyperovulation gene and pass it to his daughters. If his daughter inherits this trait, she will have the increased biological capacity to release two eggs, raising her chances of having fraternal twins. This mechanism explains why the trait often appears to “skip a generation,” as it is passed through a male carrier who does not express the trait himself.
Other Factors Increasing the Odds
Beyond genetics, several non-hereditary factors influence the statistical likelihood of conceiving twins. Maternal age is a factor, as women over 35 experience a natural rise in FSH levels just before menopause. This hormonal surge can stimulate the ovaries to release multiple eggs, increasing the chance of fraternal twins.
The number of previous pregnancies (parity) also plays a role; women who have had multiple pregnancies have a higher chance of a subsequent twin pregnancy. Ethnicity is another variable, with the highest rates of fraternal twinning observed in African populations and the lowest rates in Asian populations.
Assisted Reproductive Technology (ART), such as In Vitro Fertilization (IVF) and fertility drugs, substantially impacts twin birth rates. Medications designed to stimulate ovulation can cause hyperovulation, and transferring multiple embryos during IVF directly increases the odds of a multiple gestation pregnancy. While medical interventions have made twin births more common, genetic factors rooted in hyperovulation remain the only inherited cause.