For women one of the most important factors is age. In normal conditions, when a woman and man are healthy and younger than 35 years the chance of a pregnancy occurring each month is between 5-25%. As a woman gets older the chance of a pregnancy each month becomes lower because she has less eggs and these eggs are not such good quality.
By the age of 40 years pregnancy is becoming less common and it is almost impossible to achieve a pregnancy from the age of 45 years without using someone else’s eggs (donor egg IVF).
This reducing fertility as a woman gets older happens because a woman is born with all the eggs she will ever have. As she goes through life these eggs naturally reduce in numbers. By forty a woman has only 1-2% of the eggs she was born with. Also the good eggs tend to be used first and as you get older the eggs being released are not such good quality. This causes higher rates of both miscarriage and abnormal babies in women 40 and over.
This reducing fertility as a woman ages is illustrated in a study that looked at pregnancy rates in women receiving sperm donation. In these cases the infertility problem was due to the male partner (male factor) and the women were all without infertility problems, so there was a need for sperm donation. The study showed that among healthy women who are 25 years old, 40 per cent (40/100) will not be pregnant after six months of trying, and 15 per cent (15/100) will not be pregnant after one year. For women who are 35 years old, 65 per cent (65/100) will not be pregnant after six months of trying and 40 per cent (40/100) will not be pregnant after one year. The study also showed that 15 per cent (15/100) of women over 35 will still not be pregnant after two years. The fertility rate of these women has been halved because of age alone.