Since these racial differences appear to be based on relative levels of FSH, the effect shows up in fraternal, as opposed to identical, twins. Fraternal twins are much more common than identical twins as a general rule: In the U.S., they constitute 70 percent of all twin births, and there's every reason to believe they're the main source of the recent twin boom. However, there are indications that identical twins are on the rise as well. In England and Wales, for example, the rate started going up in 1970. That may be attributable to the use of oral contraceptives: According to one theory, women who get pregnant shortly after being on the pill end up having more monozygotic twins. It's also possible that assisted hatching, a method used in conjunction with IVF to increase the probability of implantation, alters the structure of the fertilized egg membrane in such a way that the odds of a split embryo are increased.
In any case, the graph above suggests that the development of reproductive technologies has eliminated the twinning gap between black and white mothers in recent years. The first IVF child born in the U.S. came in 1982 *, and rates of multiple births have increased in a virtually linear trend for women of all races ever since. The effect has been most dramatic among white women—surely on account of long-standing racial inequalities in education and income.
But the graph also reveals what looks to be a leveling off of twin rates in the past few years.We don't know exactly why this is happening, but it's possible that doctors have simply gotten better at performing IVF, so they're able to complete the procedure while transferring fewer embryos. The latest numbers show that in 2009, doctors implanted an average of two embryos during treatment—down from about six embryos in 2003.
Doctors may be getting better at their job, or maybe they're just showing more restraint. In countries where reproductive technology is more closely regulated, governments cap the number of embryos that can be implanted into a womb. England, for instance, allows a maximum transfer of three embryos. In contrast, American mothers make this decision with their physicians, and there are no hard and fast rules. Nadya Suleman had 12 embryos transferred—six times the recommendation for a woman her age—en route to her infamous octomotherhood. But concern over the possibility of premature babies—since multiples tend to come earlier than singletons—has led some in the U.S. to call for European-style restrictions on IVF treatment. Groups such as the Society for Assisted Reproductive Technology and the American Society for Reproductive Medicine have also created guidelines recommending best practices for embryo transfer according to a patient's age.
Whatever their motivation, IVF doctors seem to be transferring fewer embryos over time. Will that be enough to counteract the delayer boom and keep twinning rates level in years to come? We'll have to see. A twin takeover may yet be on the horizon.
Correction, Aug. 24, 2011: The article originally implied that the first IVF child born in the U.S. was also the first overall. In fact, an IVF child was born overseas in 1978. (Return to the corrected sentence.)