Why is the number of twins increasing?

The number of twin births has increased dramatically over the past 30 years. A recent government study showed that the twin birth rate rose 76% from 1980-2009! In 1980, one in every 53 babies born in the US was a twin. In 2009, one in every 30 babies was a twin! This increase is due to two factors: the tendency for women to delay having children until they are older, and the increased use of fertility treatments. Both of these factors result in an increase in fraternal (dizygotic or DZ) twins. DZ twins occur when two eggs are released at the same time and are fertilized by two different sperm.

The older a woman is, the higher 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, and an increased level may cause multiple eggs to be released.  So while older women are statistically less likely to get pregnant, if they do get pregnant, they are more likely to have DZ twins.

Older women are more likely to have trouble conceiving. And fertility problems are common among couples of any age. The use of fertility treatments has soared since 1980, and this has caused a huge  increase in DZ twinning. Fertility treatments include fertility drugs and assisted reproductive technologies.

Fertility drugs given to women to stimulate ovulation can cause hyperovulation (the tendency for multiple eggs to be released at ovulation). This can result in DZ twins.

Assisted reproductive technologies such as in-vitro fertilization (IVF) have greatly increased the rate of DZ twinning. 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, multiple embryos are transferred into her womb. This often results in twins and higher multiple births.

The increase in births to older women accounts for only about one-third of the rise in twinning over the last 30 years. The rest of the increase is due to fertility treatments such as fertility drugs and in-vitro fertilization.