Every woman is born with a fixed number of immature eggs in their ovaries which would have been pre-determined when you were developing in your mother’s womb. Various factors such as your genetics, environment and nutrition determine the number of eggs you are born with. At birth, most girls will have about one to two million eggs, and at the time of menarche, these would have gone down to about 400,000. In every menstrual cycle, around 1000 egg containing sacs (follicles) get recruited to produce one dominant follicle. A mature egg is released from this dominant follicle (ovulation), and the remaining recruited follicles which do not release the egg disappear from your ovaries as they will be re-absorbed into the body. As a result, the number of eggs decreases as the you get older. The decline is rapid after 37 years and even rapidly after 40 years. The average age of menopause in the UK is 51 years when women have about 1000 immature eggs but these generally are not fertile. Even if the ovulation does not happen regularly due to using contraceptive pill, or when pregnant/breastfeeding, the number of eggs continues to decline at the same rate.
It is not just the number of eggs that decline with age, but the quality of eggs will also be poorer as you become older. The chromosomal abnormalities in the eggs are higher in women on either end of their reproductive life, and it is estimated that around 75% of the eggs will be chromosomally abnormal at the age of 40 years. Poor quality eggs result in chromosomally abnormal embryos which either do not implant into the womb cavity or if they implant, there will be a higher chance of resulting in a miscarriage or a baby being born with chromosomal abnormalities.
All other things being equal, your age is the important factor that determines your fertility. There is a decline in female fertility starting in mid-thirties reflecting in the decline in number and quality of eggs, with lower fertility especially after the age of 35 years. The fertility will continue to reduce every year, whether or not you are fit and healthy. The rate of decline in a your fertility will depend on various other factors such as genetic make-up and lifestyle (e.g. smoking cigarettes, being overweight and obese) factors, gynaecological conditions, previous operations on the ovaries, etc.
At JIVA Fertility, we offer various tests to check for egg reserve markers and offer appropriate advise related to fertility.
