Two primary fertility hormones control ovulation development in women and sperm development in men. Any imbalance or deficiency of these hormones can cause infertility.
Fertility drugs stimulate the production of these critical hormones and can help otherwise infertile couples to conceive. However, they also contribute to the growing number of multiple births (twins or higher) in the U.S., which have a higher risk of pregnancy complications, including pre-term labor.
One-third of all twins born are identical. They begin as one fertilized egg that then divides in half. Fraternal twins are the result of two different eggs being fertilized by two different sperm. Triplets or higher births can result from three or more eggs being fertilized, one egg splitting more than once, or a combination of both.
Women over 30 are more likely to conceive multiple babies even without fertility treatment. This happens when more than one egg is released by the ovaries during a reproductive cycle. Fertility drugs are designed to stimulate hormones that cause the ovaries to release even more eggs, greatly increasing the chances of a woman conceiving more than once.
During the year 2002, in the U.S. there were 132,535 live multiple births. Of those, 125,134 were twin births; 6,898 were triplets; 434 were quadruplets; and 69 were quintuplet and higher births.