Since the 1970s more and more twins have been popping up across the developed world (large graph). Women have been having babies at older ages, so more are becoming pregnant at the natural peak age for twins: about 35. And fertility treatments have become more common, which boosts the chances of twins further (bars above graph). Because multiple births are dangerous for both mothers and babies, public health officials would like to reverse the trend. Sweden, Denmark and a few other countries are turning their twinning rates around with guidelines or regulations that restrict the number of embryos implanted during most in vitro fertilization (IVF) procedures (small graphs). That change has also cut the rate of triplets significantly.

 
Credit: KATIE PEEK; Sources “Twinning Rates in Developed Countries: Trends and Explanations,” By Gilles Pison et al., In Population and Development Review, VOL. 41, No. 4; December 2015 (large graph); “Fertility Treatments and Multiple Births in the United States,” By Aniket D. Kulkarni et al., In New England Journal of Medicine, VOL. 369, No. 23; December 5, 2013 (structure of small graphs); National Center for Health Statistics, 1971–2014 (data in small graphs)