Birth rates for MZ twins are consistent among all races (about 4 per 1000); but the incidence of DZ twinning varies among races (8 per 1000, among Caucasians, 16 per 1000 among people of African descent, and 4 per 1000 among Asians). A genetic predisposition or inherited characteristic for DZ twinning exists in some families, but the consistency of MZ twinning among all populations suggests that identical twinning is a random occurrence that is not influenced by genes. Overall, about 1 in 80 births in Australia is a twin and of these about 30% are MZ.
A dramatic increase in the number of DZ twin, triplet and quadruplet births occurred when new treatments for infertility, most involving the use of hormones to stimulate the ovulation of more than one egg, were developed in the late 1970s. In treatments where the mature eggs are harvested and fertilized outside of the woman’s body, as is the case with IVF (In Vitro Fertilisation), two or more embryos have routinely been transferred back into the uterus in order to better the odds that at least one will implant successfully. Surprisingly, treatment with ART (Assisted Reproduction Technologies) also seems to boost the rate of MZ twinning, but at this stage researchers don’t understand why. Multiple pregnancies are high risk, especially where supertwins (more than two) are involved. Many leading infertility specialists now advocate the transfer of only one embryo at a time, and certainly never more than two, since advanced techniques have improved the chances of a viable pregnancy resulting from the transfer of just one high quality embryo. As this becomes accepted practice, there is likely to be a decline in the rate of ART related twin and supertwin births.