(Reuters) - U.S. researchers have developed a formula that can predict whether fertility treatment will succeed more accurately than using age alone, and used it to develop a commercial test.

They said their test, published in the Proceedings of the National Academy of Sciences on Monday, could save couples the agony and expense of multiple attempts to have babies using in vitro fertilization, also known as IVF.

Surprisingly, they said, the test showed that couples who would have been discouraged from trying again using traditional assessments were actually likely to succeed.

In IVF, egg and sperm are united in a lab dish and resulting embryos are implanted into the mother's uterus to grow. Predicting whether it will work is tricky and doctors rely heavily on a woman's age.

A team lead by Dr. Mylene Yao of Stanford University in California, decided to look at dozens of factors, including age but also looking at how well and how fast the embryos grow, a woman's hormonal response to the treatment and the condition of her uterus when the embryo is implanted.

They used data from more than 5,000 IVF cycles performed at Stanford Hospital from 2003 to 2008 and matched it to success rates.

"Since this model uses clinical data from a previously failed IVF attempt, a first IVF treatment can be viewed both as an infertility treatment and as a potential prognostic tool for future cycles," Yao said in a statement.

As many as 80 million couples worldwide and 7.2 million in the United States are infertile. The infertility market is large and growing with an estimated 140,000 IVF cycles in the United States in 2008.

That accounts for about 2 percent of infertile couples, Yao's team said. About 1 percent of U.S. newborns were conceived using IVF, the researchers said, and a million babies have been born using IVF globally since Louise Brown, the first "test tube" baby, was born in Britain in 1978.

Yao and colleagues have founded a new company called Univfy to develop and commercialize the test. The company has licensed the technology from Stanford and has applied for a patent. It is seeking U.S. Food and Drug Administration approval to market the test by this fall.

(Editing by Doina Chiacu)