A dozen eggs might be just the right amount to buy at the grocery store, but when harvesting human eggs for in vitro fertilization (IVF), 15 is the magic number, resulting in the greatest chances of a live birth, according to a new study.

The results, based on a computer model using data from women receiving IVF treatment in the United Kingdom, strike a balance between harvesting too few eggs and reducing the chances of a live birth with harvesting too many eggs and risking the potential of a negative health outcome for the mother.

"For a woman under 34, the live birth rate with 15 eggs is 40 percent," said study researcher Dr. Arri Coomarasamy, a clinical reader and consultant in reproductive medicine and surgery at the University of Birmingham in England. However, with five eggs harvested, the live birth rate is 30 percent and with 40 eggs harvested, the live birth rate is 33 percent, he said.

The findings are especially important for the hundreds of thousands of women who rely on the procedure to become pregnant , he said.

In the United States, there were 148,055 assisted reproductive technology procedures (which includes IVF) in 2008, which resulted in 46,326 live births, according to the Centers for Disease Control and Prevention. Use of assisted reproductive technology has doubled in the last decade, and more than 1 percent of infants today are conceived with the technology.

The study was published online today (May 10) in the journal Human Reproduction.

Just the right amount of eggs

Coomarasamy and his colleagues analyzed health data from 400,135 IVF cycles between April 1991 and June 2008.

Then, researchers created a model that takes into account the woman's age, the number of eggs doctors retrieved from her ovaries and her success at giving birth to a baby.

They found that 15 was the optimal number of eggs to retrieve from the woman's ovaries without putting the woman at increased risk for ovarian hyperstimulation syndrome, a painful condition that occurs when the ovaries are stimulated to produce too many eggs that results in leaked fluid into the belly and chest.

The syndrome, which affects about 10 percent of women undergoing IVF, can lead to significant weight gain, severe pain in the abdomen and bloating, according to the National Institutes of Health.

Retrieving 20 or more eggs is associated with an increased risk of ovarian hyperstimulation syndrome, Coomarasamy said.

Why 15?

Women who produce too many eggs may have poorer quality eggs, leading to the lower live-birth rate, Coomarasamy said.

"Many women who produce too many eggs have polycystic ovary syndrome [where no eggs are released during ovulation] and polycystic ovary syndrome may be related to poorer egg quality," he told MyHealthNewsDaily. "However, we do not have a comprehensive understanding for why live birth should drop with egg numbers [greater than] 20."

Right now, eight to 14 eggs are typically retrieved from a woman's ovaries for IVF, said Dr. George Attia, associate professor and director of the Infertility Center at the University of Miami Miller School of Medicine.

However, the number of harvested eggs should depend on the woman who is undergoing the IVF procedure: her age, her hormone levels and other factors, Attia said.

The people in the study who had 20 or more eggs harvested were probably overstimulated, which increases body levels of estrogen tremendously, he said. Increased estrogen levels decrease the opportunity for eggs to implant, thereby leading to a decreased chance of pregnancy and birth.

Pass it on: To optimize the chance of having a live birth with in vitro fertilization, doctors should harvest 15 eggs from a woman's ovaries.

Follow MyHealthNewsDaily staff writer Amanda Chan on Twitter @AmandaLChan.