During 2009, one in every 30 babies born in the United States was a twin. This is the result of a steady, yet strong growth, in the incidence of twins over the last three decades. Historically, the incidence of twins in the United States was at a rate of one in 53.
Reasons for the increased incidence of twins has been attributed to mainly two factors:
- The majority of women are now delaying starting their families until they are in their thirties. Statistically speaking, women in their thirties are more likely to have twins than in any other age group.
- The increase in the usage of ART (Assisted Reproductive Therapy) such as IVF, which can involve the transfer of two embryos in one IVF cycle. This is reported to be the greatest contributing factor accounting for two thirds of the increase.
Analysis in different demographics resulted in the following conclusions:
- Women over the age of 40 had the greatest increase in twin births. This has been attributed to their reliance on IVF.
- The incidence of twins increased in all of the US mainland states.
- Connecticut has the highest incidence of twins with nearly one in 20 births being a twin.
- All ethnic groups experience an increase. White, black, and Hispanic women all had more twins. However, due to their better access to IVF, white women experienced the greatest increase.