The optimum number of eggs to harvest from women undergoing fertility treatment is 15, experts have discovered. The magic number, determined by a major study looking at 17 years of data, gives the best chance of a live birth. Taking more than 15 eggs is not only less safe, but no more effective, it found. The study 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. The chances of success level off between 15 and 20 eggs and then start to fall. As more eggs are taken, there is a growing risk of complications from over-stimulation of the ovaries during  drug treatment. Ovarian hyperstimulation syndrome can cause pain, weight gain, and occasionally dangerous blood clots and oxygen starvation. In severe cases, it can be fatal. The study, reported in the journal Human Reproduction, is the first major investigation of egg numbers in IVF treatment and live birth rates. One or two embryos are usually returned to the womb with the remainder frozen as eggs or embryos for use in repeat attempts.