A new finding by researchers in Ottawa shows that when it comes to fertility, success is related to the size of the egg.

Scientists at the Ottawa Health Research Institute have discovered the mechanism that controls the size of human eggs and early embryos.

This is significant, because size is a major factor in the success of in vitro fertilization (IVF).

"It seems to be very important for eggs to be exactly the right size and when they shrink too much or they're the wrong size they fail to develop," said Dr. Jay Baltz, a senior scientist at the institute.

Conditions inside a woman's body allow eggs to maintain their right size.

In the lab, scientists use trial and error to adjust the volume of the egg. The new finding should help eliminate some of the guesswork involved in IVF.

The researchers have discovered that something is missing when fertilizing an egg outside the body - a small amino acid called glycine.

This discovery, said Baltz, may not only give women a better chance at having a baby, but will also address the ethical problem of implanting too many embryos.

"The ultimate goal is to be able to put a single egg and get a single baby," he said.

"This is one thing that will lead us in that direction. What you really want to do is be able to know that the embryo that you grow in the clinic is healthy and has a high probability of producing a baby."

Only 10 to 20 per cent of eggs fertilized outside the body go on to produce pregnancies, he said.

The research was sponsored by the Canadian Institutes of Health Research. The study appears in this month's issue of the Proceedings of the National Academy of Science.