Why does fertility decline with increasing age?
Ageing of the ovaries is the most prominent factor and is part of the normal ageing changes that affect all organs and tissues. Most women have about 300,000 eggs in their ovaries at puberty. For each egg that matures and is released (ovulated) during the menstrual cycle, at least 500 eggs do not mature and are absorbed by the body. By the time the woman reaches menopause which usually occurs between 50-55 years, there are only several thousands eggs remaining. As the woman ages, the remaining eggs in her ovaries also age, making them less capable of fertilization and their embryos less capable of implanting.
Fertilization is associated with a higher risk of genetic abnormalities e.g. chromosomal abnormalities such as Down syndrome with increasing age. The risk of a chromosomal abnormality in a woman age 20 years is 1/500 while the risk in woman age 45 is 1/20.
Gynaecological problems such as pelvic infection, tubal damage, endometriosis, fibroids, ovulation problems etc tends also to increase with age. As the woman gets older, she has more time to develop these conditions, which will adversely affect her fertility.
Sexual function is also decreased with ageing e.g. libido, frequency of intercourse etc.
The effect of ageing in endometrial receptivity (ability of the endometrium to receive the embryo) is controversial. There is increasing evidence that the receptivity decreases with age.
Ageing does not just affect women, but also men to a much lesser degree. It affects sperm and coital frequency. There is no maximum age at which men are not capable of conceiving a child.
Advanced maternal age increases the risk of autosomal dominant diseases such as Marfans syndrome, neurofibromatosis and achondroplasia.