It could be because the man has a low sperm count, or low sperm health (for example, they don’t swim very well), or it could be because the woman is not producing high-quality eggs that the man’s sperm can fertilise. There are lots of possible reasons, and lots of things that can be done to increase fertility in men and women, such as living a healthy lifestyle.
There are lots of things that can effect fertility such as quality (as Ian says) or there could be physical barriers such as scar tissue in the fallopian tube, or problems with the implantation of an egg into the uterus. The most common reason for infertility is old age!! Many women are waiting until our mid/late thirties before trying to have children and fertility is rapidly reducing at this age.
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slattercwh commented on :
Elliot =)