Limited Number of Eggs
After 20 years of age, female fertility begins to gradually decline. This is partly because a woman is born with a limited number of eggs (approximately 300,000). Generally, only few hundred of these eggs will ovulate during a woman’s reproductive lifespan. The others, and the vast majority of a woman’s eggs, die off continuously in a process called atresia.
Furthermore, eggs produced earlier in a woman’s lifetime have a higher energy supply than the eggs that ovulate in the later reproductive years of a woman’s lifetime. This energy (and a lot of it) is needed for the egg to effectively complete fertilization and to control the chromosomes and genetic material during meiosis.
Declining Egg Quality
The dominant factor that causes age related reduced fertility is the decline in egg quality.Numerous studies show that there is a consistent decline in fecundity (or the ability to produce offspring) after age 30 to 35. It is twice as hard for a woman to conceive at age 35 as it is at age 25. For women over 40 years of age, involuntary infertility affects almost 65% of women.
As a woman ages, her monthly fertility rates (fecundity) decrease and her possibility of miscarriage also increases. This decline is marked by an increased rate of chromosomal abnormalities in resultant embryos, an acceleration in follicular atresia, and decreased ovarian reserve. Doctors suggest that increased miscarriage rates in older women are largely due to the increased probability for genetic abnormalities in resultant embryos.
IVF Rates and Age
Both age and ovarian reserve play a specific role when it comes to predicting a woman’s response to fertility hormone stimulation. A woman of advanced reproductive age with a normal ovarian reserve marker still has to contend with age related decline in fertility due to diminished egg quality. Because in vitro fertilization (IVF) success rates are proportional to the number of eggs that are retrieved, fertilized, and transferred, age-related low responders with poor follicular development have diminished pregnancy rates.
There has been an unprecedented amount of couples opting for infertility treatment after delaying childbearing into their late thirties and forties, for a large number of reasons. The impact of aging on fertility has become increasingly relevant as one out of every five American women is having her first child after the age of 35, a 50% increase in the last decade.