Fertility improvement techniques and pregnancies late in life increase the chance of multiple births. Fertility techniques such as [in vitro] imsemination were previously unknown; fertility drugs enhance ovulation; and older women release more eggs up until menopause.
Fertility treatment has led to a large increase in multiple births. In the UK the number of triplets has doubled in the decade to 1990. The number of women in the USA giving birth to 3 or more babies at once quadrupled between 1980 and 2000. The number of multiple live births per 100,000 live births increased by 252% for whites and 52% for blacks. The rate of twins rose 30% in the same period. The overall rate has been rising by 11% each year. In 1994, there were 4594 such births, most of them to white married college-educated women beyond 30 years of age.
1. Multiple births beyond triplets can and ought to be avoided.