Infertility is a challenge faced by a growing number of couples. In vitro fertilization (IVF), a process by which egg cells are fertilized by sperm outside the womb in a controlled environment, is now a standard of treatment when all other methods of assisted reproductive technology have failed. But it is not an exact science and there are variables which affect the success rate. But new research out of the U.K. has narrowed those variables down, and found that harvesting 15 eggs will give a couple the optimum chance of conceiving.

Over 1 million babies have been born because of successful IVF procedures, with over 113,000 IVF treatment cycles performed in the United States annually, a figure that continues to grow at a steady pace. The process is costly and on average requires seven cycles of treatments, which can be physically and emotionally challenging. For each procedure, a woman undergoes several weeks of hormone injections, to stimulate the ovaries, and have their eggs extracted, to be fertilized in the lab, in order to produce viable embryos. If viable embryos are obtained, then they are placed into the mother’s womb.

The possibility of a continuing pregnancy being achieved by IVF has improved dramatically from a nearly 0 percent chance to 1 chance in 4 to 6 at IVF centers worldwide. And with new research, such as this study published in the journal Human Reproduction, the chances of success can only increase.

The study, analyzed data from more than 400,000 IVF cycles conducted in the U.K. between 1991 and 2008, found that the number of live births declines when fewer numbers of eggs are retrieved from a woman’s ovaries during a cycle of IVF and that the chances of success level off between 15 and 20 eggs and then start to fall.

“Our data show that around 15 eggs may be the best number to aim for in an IVF cycle in order to maximize the chances of a live birth while minimizing the risk of ovarian hyperstimulation syndrome (OHSS), which is associated with a high number of eggs, usually over 20,” Dr. Arri Coomarasamy, a clinical reader and consultant in reproductive medicine and surgery at the University of Birmingham, said in a journal news release.

“Mild stimulation protocols aim to retrieve less than six to eight eggs; a standard stimulation should aim for 10-15 eggs, and we believe this is what is associated with the best IVF outcomes,” said Coomarasamy. “When the egg number exceeds 20, the risk of OHSS becomes high.”