Researchers have found that clomiphene, the treatment for couples with unexplained infertility, results in more live births than its alternative, letrozole, UPI reports.

Drugs used to help couples with unexplained fertility stimulate a woman's ovaries to release an egg. The doctors then insert the man's sperm directly into the uterus.

"Ovarian stimulation medication promotes pregnancy by increasing the number of eggs that a woman ovulates and by enhancing implantation through hormonal effects in the endometrium," said Dr. Ruben Alvero, director of the Division of Reproductive Endocrinology and Infertility at Women and Infants Hospital of Rohode Island, in a press release.

"Unfortunately, ovarian stimulation can be complicated by ovarian hyperstimulation syndrome, which results in multiple gestations with increased risk of preterm birth."

Researchers had started considering the drug letrozole based of anecdotal evidence that it helped women conceive with less risk of multiple births.

The researchers, part of the National Institute of Child Health and Human Development's Cooperative Reproductive Medicine Network, worked with 900 women between the ages of 18 and 40 at 12 facilities.

As part of the study, 300 women were given clomiphene, 299 were given letrozole, and the remaining 301 were treated with gonadotropins, substances produced by pituitary gland that cause the ovary to release an egg. Pregnancy occurred in 35.7 percent of those receiving clomiphene, and in 28.4 percent of those who got letrozole.

Clomiphene also resulted in the fewest multiple pregnancies, followed by letrozole and gonadotropins.

"The conclusion for couples with unexplained infertility is that clomiphene probably still remains the first-line therapy," Diamond said.

The study is published in the New England Journal of Medicine.