Women who delay starting a family have been warned it is 10 times harder to get pregnant at 43 than at 37.
Researchers found that, on average, a 37-year-old needs to produce 4.4 eggs to produce one normal embryo, but at 43, she needs to produce 44 eggs, meaning it will take her almost four years to conceive – rather than only four months.
Researcher Dr Meredith Brower told a conference that the number of eggs needed for a viable pregnancy rises 'almost exponentially' after 42, and urged women to freeze their eggs without delay.
Dr Brower, of the University of California, Los Angeles, analysed data on 198 women aged between 20 and 45 having IVF treatment.
This included how many eggs they produced, and the health of the embryos produced after they were fertilised in a dish.
Not surprisingly, the younger women produced more eggs – and more genetically normal embryos. Dr Brower calculated that a woman under 35 would on average need just 3.8 eggs to make one healthy embryo.
Between the ages of 35 and 37, 4.4 eggs would be needed. The figure starts to climb at 38. Women aged between 38 and 40 need 9.4 eggs and women aged 41 to 42 need 10.1.
The number then soars, as fertility falls, with women aged over 42 needing 44 eggs.
It's thought the results apply to women trying to conceive naturally.
Last week it was revealed that nearly half of all British babies are born to women aged 30 and older, and the number born to women in their late thirties has almost trebled in recent years.
More on Parentdish
Apple and Facebook offer employees the chance to freeze their eggs
Older mums outnumber younger mothers for the first time