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The odds of achieving a live birth after in vitro fertilization (IVF) increased steadily with the number of eggs retrieved after ovarian stimulation, topping out at about 15, data from a large IVF registry showed.

The estimated live birth rate reached 40% after retrieval of 15 eggs from women ages 18 to 34, as reported online in Human Reproduction.

The effect of age took over from there, as live birth rate associated with 15 eggs decreased as women's age increased, falling to 16% for women 40 or older.

The live birth rate improved no further with more than 15 eggs and declined steadily across all age groups with retrieval of more than 20 eggs, Sesh Kamal Sunkara, MD, of King's College London, and colleagues reported.

"The relationship between the number of eggs and live birth, across all female age groups, suggests that the number of eggs in IVF is a robust surrogate outcome for clinical success," they wrote in conclusion.

"The results showed a nonlinear relationship between the number of eggs and live birth rate following IVF treatment. The number of eggs to maximize the live birth rate is approximately 15."

The number of eggs retrieved after ovarian stimulation is considered a key prognostic variable for successful live birth after IVF. However, the relationship between the number of eggs and live birth remains poorly understood, the authors noted in the introduction to their findings.

Previous studies of the relationship have come from small, single-center experiences and have yielded conflicting results. Moreover, none of the studies reported live birth rate, instead focusing on pregnancy rates, the authors continued.

To augment the limited amount of data on the issue, Sunkara and colleagues used a nationwide database to examine the association of egg retrieval with live birth involving 400,135 IVF cycles. The analysis covered the time from 1991 to 2008.

Half of all IVF cycles involved women ages 18 to 34, declining to 24.3% among women ages 35 to 37, 12.8% among women ages 38 to 39, and 12.6% among women 40 or older.

The median number of eggs retrieved was nine, resulting in a median of five embryos. The overall live birth rate was 21.3%. The live birth rate increased over time from 14.9% during 1991 to 1995 to 26.5% during 2006 to 2008.

Investigators identified a strong association between the number of eggs retrieved and the live birth rate up to about 15 eggs. Thereafter, the association plateaued and began to decline with retrieval of more than 20 eggs. Analysis of the data by four-year intervals showed the same pattern of association across the entire study period.

For a given number of eggs, the live birth rate increased over time but decreased with increasing age. However, the live birth rate increased globally across all age groups over time.

"Our data suggest that around 15 eggs may be the optimal number to aim for in a fresh IVF cycle in order to maximize treatment success while minimizing the risk of ovarian hyperstimulation syndrome, which is associated with high number of eggs of greater than 18," the authors wrote in their discussion.

"The decline in the live birth rate observed with higher number of eggs could be due to the deleterious effect of the raised serum estradiol levels affecting embryo implantation."

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