The maximum number of eggs to harvest from a woman's ovaries undergoing fertility treatments is 15, say researchers.
Investigators led by Arri Coomarasamy of the University of Birmingham in central England looked at data from more than 400,000 IVF cycles in Britain between 1991 and 2008.
Harvesting around 15 eggs in one go gave the best statistical chance of a live birth, they found.
This finding is in line with middle-range doses of powerful hormones to stimulate egg production, they found.
The chances of a baby level off between 15 and 20 eggs and then steadily decline beyond 20 eggs.
The success rate for 15 eggs in 2006-2007 was 40 percent among women aged 18-34, 36 percent for those aged 35-37, 27 percent for those aged 38-39 and 16 percent for women aged 40 or more.
The findings should be a useful tool for fertility doctors who want to make IVF as efficient as possible, avoid wasting precious eggs and limit the risk of dangerously overstimulating the ovaries, say the authors.
Previous research has looked at the link between egg numbers and pregnancy rates, but this is the first to explore the outcome in terms of live births.
The study appears in a European journal, Human Reproduction.
"Live birth" was defined as an event in which at least one baby was born alive.
IVF treatments entail taking eggs and fertilising them in a lab dish with sperm. Several early-stage embryos are then implanted in the uterus in the hope that one will result in a live birth, although multiple births sometimes result.
Embryos that are not immediately implanted are usually frozen, so that they can be thawed for future IVF attempts.
The data used in the study did not distinguish between live births that resulted from fresh or thawed embryos.