Development of twins

In most pregnancies a single embryo develops in the uterus, but in some cases, two embryos develop together. These are called twins.

Most twins are fraternal or dizygotic twins, meaning that they originate from two separate eggs that are fertilized individually.

A minority are identical or monozygotic twins, meaning that they originate from a single zygote that quickly splits into two separate groups of cells.

Fraternal twins are from two separate eggs that are fertilized by different sperm, so they have completely separate genetic makeups.

They don’t look any more or less alike than regular siblings, although the resemblance can still be very close—you may be surprised to learn that Mary-Kate and Ashley Olsen, for example, are fraternal twins, not identical twins.

Fraternal twinning occurs at a rate of about 10 per 1000 births worldwide.

Most of the time, fraternal twinning happens when the ovaries release two eggs simultaneously, which is called hyperovulation, instead of releasing one egg at a time.

Research suggests that some mothers of fraternal twins may produce an overabundance of a hormone called follicle-stimulating hormone, or FSH, which stimulates the growth of ovarian follicles.

People who become pregnant with fraternal twins tend to be taller and heavier on average, with shorter, more frequent menstrual cycles, all of which are characteristic of having high levels of follicle-stimulating hormone.

Because follicle-stimulating hormone levels gradually rise with age, fraternal twin pregnancies become increasingly likely in people aged 35 or older, and this also helps explain why parents who have given birth to fraternal twins once are more likely to do so again.

The likelihood of having fraternal twins resulting from hyperovulation is thought to have a genetic component, but no specific gene has been identified yet.

Identical twins are less common than fraternal twins, occurring at a rate of about 4 per 1000 births worldwide.