Scottish scientists have finally figured out why female fertility drops off so rapidly with age: a woman’s “ovarian reserve,” or the potential number of eggs she is born with, declines by almost 90 per cent by age 30. The study from the University of St Andrews and Edinburgh University found that most women are born with 300,000 potential egg cells but by age of 30, only 12 per cent are left on average; by age of 40, the reserve is down to just three per cent. Though women continue producing eggs throughout their 30s and 40s, the reservoir of potential eggs from which they are taken shrinks to almost nothing. And though the body chooses the best eggs from the reserve, the likelihood is that the quality of the eggs will suffer with age, increasing the difficulty of conception and the risk of an unhealthy baby. The lesson, says study co-author Dr Hamish Wallace, is that women shouldn’t wait too long to try to conceive: “Our research shows that they are generally over-estimating their fertility prospects.