The twin rate in the U.S. has skyrocketed

The twin boom can be explained by changes in when and how women are getting pregnant. Demographers have in recent years described a "delayer boom," in which birth rates have risen among the sort of women—college-educated—who tend to put off starting a family into their mid-30s or beyond. There are now more in this group than ever before: In 1980, just 12.8 percent of women had attained a bachelor's degree or higher; by 2010, that number had almost tripled, to 37 percent. And women in their mid-30s have multiple births at a higher rate than younger women. A mother who is 35, for example, is four times more likely than a mother who is 20 to give birth to twins...

Better-educated, richer parts of the country do produce more twins than anywhere else. Connecticut, for example, is the wealthiest state in the union (its residents made an average of $56,272 per capita in 2008) and it ranks in the top three when it comes to education (35.6 percent of its residents held a bachelor's degree). It's also in the top three for the availability of IVF clinics, with 2.52 per 1 million residents. And its twinning rate—43 for every 1,000 live births—is third in the nation...

...it turns out that black mothers are more likely to have twins than those of any other racial group. It has been known for decades that levels of FSH production differ among subpopulations in the U.S. and overseas. Black women in African countries produce more FSH than anyone else, and they have the highest fraternal twinning rate in the world. Women in East Asian countries, on the other hand, have the lowest FSH-levels and produce few twins. 

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