The number of twin births has increased dramatically over the past 40 years. A study of US birth records 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. In the past, to increase the chances of a successful pregnancy, multiple embryos were transferred, which often resulted in twins and higher multiple births. However, recently updated guidelines from the American Society for Reproductive Medicine and Society for Assisted Reproductive Technology recommend that only one embryo be transferred, as multiple births are associated with more risks in pregnancy.