Twenty-five years ago, almost no one had a cell phone. Very few of us had digital cameras, and laptop computers belonged only to the very rich. But there is something else—not electronic, but also man-made—that has climbed from the margins of the culture in the 1980s to become a standard accoutrement in upscale neighborhoods across the land: twins.
According to the latest data from the Centers for Disease Control and Prevention the U.S. twin rate has skyrocketed from one pair born out of every 53 live births in 1980 to one out of every 31 births in 2008. Where are all these double-babies coming from? And what's going to happen in years to come—will the multiple-birth rate continue to grow until America ends up a nation of twins?
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. That seems to be on account of her producing more follicle-stimulating hormone, or FSH, which boosts ovulation. The more FSH you have in your bloodstream, the greater your chances of producing more than one egg in each cycle, and having fraternal twins as a result.
Older mothers are more likely to have twins, but they're also more likely to use in vitro fertilization or other reproductive technologies in order to get pregnant. IVF increases twinning rates, too. Treatments begin with "ovulation induction," in which FSH or other drugs are prescribed to the would-be mother, to help her release multiple eggs in a given cycle. That way the doctors can try to maximize their chance of success by creating a whole bunch of fertilized embryos and then transferring several of them at a time into the mother's uterus. Something like 25 to 30 percent of women treated with IVF end up having multiple births.
The two factors described above—delayed motherhood and in vitro fertilization—are, of course, related: The longer you wait to have kids, the more likely it is that you'll need IVF; and the easier it is to get IVF, the more likely it is that you'll wait to have kids. There's a socioeconomic link, too: The well-educated women who tend to put off starting a family are also the ones most able to afford a $20,000 fertility treatment. One can deduce, then, that twin rates across the country should correspond both with rates of educational attainment and average income. For the most part, the following maps support that hypothesis.
(Compare twin rates with nationwide trends in the widget below. Select a national dataset from the dropdown menu on the right, and compare it with the pattern of twin birthrates shown on the left.)
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.
Now look at New Mexico, which is near the middle for education (24.7 percent of its residents hold a B.A. or higher) and ranks near the bottom for average income ($33,430), with very few IVF clinics around (0.49 per million residents). Its twinning rate is the lowest in the United States, at 24.7 per 1,000 live births.
But there are some places where the theory breaks down. Consider the Deep South, where five of the poorest states in the country consistently produce above-average numbers of twins. The mean income across the belt running from Louisiana to South Carolina is just $33,630 (barely higher than New Mexico's), and the average rate of college education is a meager 22.6 percent; IVF clinics are few and far between. Yet the combined twinning rate for these states is 32.9, which far exceeds New Mexico's, and other states with similar socioeconomic profiles. In fact, the twinning rates in the Deep South match up with those of pretty well-to-do states like Nebraska, North Dakota, and South Dakota. What's causing this twins paradox?
Along with Maryland and the District of Columbia, those states have the highest proportions of black residents in the nation, and 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. (In the U.S., twins are significantly less common among Latinos than among whites or blacks, which may help to explain the low twinning rates in New Mexico, Arizona, and Texas.)
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