A new study by researchers at the University of Iowa found that women who had only one empbryo implanted didn't have that much lower probability of becoming pregnant than those with more than one.
From 1980 to 2004, the rate of twins rose 70%. This, as one would expect, is most likely because of an increase use of fertility drugs and use of in vitro fertilization (IVF). In 2006, the twin rate was 32.1 twins per 1,000 births (3.2%) in the US. Interestingly, the Yoruba in West Africa has the highest twin rate at 45 to 50 per 1,000 (4.5 to 5.0%). This is thought to be because of yams which are an important part of their diet has a naturally occuring phytoestrogen causing the ovaries to release an egg from each ovary.
IVF experts have been pushing for using few embryos, and preferably a single one, as it reduces the complications associated with multiple and premature births.
Even still, many patients still request multiple embroyos when undergoing IVF treatment. This is inspite of knowing that a single embryo has the same chances as multiples.