Global epidemiology of multiple birth.

Multiple births, which account for approximately 3% of births and 14% of infant deaths, are increasing in frequency. Multiple birth rates began to decline in the 1950s, reaching a minimum in the 1970s and rising since then. Both twin and triplet rates followed the same rising trend until 1998, after which triplet birth rates began to decline while twin birth rates continued to rise. Rising maternal age is associated with rising frequency of dizygotic twinning up to 37 years of age. Older maternal age, associated with the social trend to delayed child bearing, accounts for 25- 30% of the rise in multiple birth rates since 1970. The resulting rise in the prevalence of infertility has given rise to unprecedented use of ovarian stimulation treatments that stimulate the development of multiple oocytes. Assisted reproduction technology and ovulation stimulation with clomiphene citrate or gonadotrophins without assisted reproduction account for similar proportions of both twin births (20- 30%) and triplet births (30- 40%). The fall in triplet rates since 2000 is reassuring, but fetal reduction of high order pregnancies may be a factor in rising twin rates. Continuing attention is needed to all possible means of minimizing triplet pregnancies.