U.S. twin births jumped sharply across three decades
Twins are two offspring from one pregnancy—monozygotic when one zygote splits, dizygotic when two eggs fertilize. Between 1980 and 2009 American twinning climbed about seventy-six percent, from roughly 9.4 twin sets per thousand births to about 16.7.
Fraternal twins arise when two eggs are fertilized by two separate sperm; they share about half their genes, like any siblings, and a boy–girl pair is the most common combination. They are the more frequent type in humans. Identical twins come from a single zygote that splits, and they occur at a steady 3 to 4 per 1,000 births in every population. A 2007 theory drawn from IVF work suggests they may also form when a blastocyst holds two inner cell masses. Very rarely, twins share a mother but have different fathers.
Fraternal twinning varies widely. The Yoruba have the world’s highest rate, 45–50 sets per 1,000 live births, perhaps because of a yam containing a phytoestrogen. Japan sits near six per thousand, and South America and South and Southeast Asia see only six to nine sets. The tendency runs through the mother’s family, doubles in mothers over 35, and rises with fertility treatment; IVF deliveries produce nearly 21 twin pairs per 1,000.
Twin pregnancies average 37 weeks, three weeks short of full term. Because male fetuses are likelier to die in the womb, and twins face higher mortality there, female twins outnumber male twins. In the United Kingdom about one pregnancy in 65 yields a multiple birth.
Identical twins are not quite identical. They have different fingerprints, and a 2013 study of 66 pairs estimated roughly one DNA difference per 12 million nucleotides in blood cells. Epigenetic differences grow with age: in a study of 80 pairs, 50-year-olds showed over three times the differences of three-year-olds, and twins raised apart differed most. Children of identical twins test genetically as half-siblings rather than cousins.
Source: Twin
