According to a study published early February 2016 in the Population and Development Review, the twinning rate almost doubled in developed countries over the last four decades.
Two main factors account for this phenomenon according to the study:
1° The expansion of medically-assisted reproduction (MAR), which has known a strong development during this period. As well as ovarian stimulation, in-vitro fertilization (IVF) techniques often go with the placement of multiple embryos, a procedure which carries the risk of multiple pregnancies. When the MAR started, obtaining twin pregnancies was seen as a success; whereas nowadays, a low rate of twin pregnancies has become a good criterion as to evaluate the MAR center’s performances.
2° Delayed childbearing, as older women tend to have twins more frequently: if fertility lowers after 35 years old, paradoxically poly-ovulation is more frequent. Indeed, if the risk of multiple births is lower when the mother is 20 years old, it reaches its maximum at the age of 37 (15 births out of 1000). Age increases the level of FSH, the hormone which stimulates follicles and the liberation of ovocytes. Moreover, another factor linked to age could be that the older ovocyte is more fragile and divides more readily at an early stage after fertilization, multiplying the possibility of developing identical twins.
The increase of multiple births continues, especially in France, the United States and the United Kingdom, and gives rise to public health problems. Indeed, these pregnancies present more risks, as much for the baby as for the mother. The biggest risk, often associated with induced deliveries, is premature babies with a higher infant mortality rate. Concerning the mother, there is a higher risk for gestational diabetes and postnatal depression.
The MAR centers lean more and more their recommendations towards single embryo transfer, and to retry in case of failure, after having frozen the other embryos.