Why does female fertility decrease with age?

In many countries around the world, the age of a mother at the birth of her first child is increasing. And with this increasing maternal age comes an increased risk of fertility issues, as well as an increased risk of chromosomal abnormalities. Yet, paternal age (father’s age) doesn’t have such an effect. Why is this the case? Read on to find out.

Sperm production versus egg production
Males can produce more than 100 million sperm a day, with an average ejaculate containing more than 40 million sperm (1).

In contrast, females have a peak number of oocytes (immature eggs) while they themselves are only 20-week-old fetuses. By birth, the number of viable oocytes has already dropped to 1–2 million, and by puberty, there are usually only 300,000–500,000 viable oocytes remaining. Throughout a women’s reproductive life, she will ovulate 400–500 oocytes and the remainder will be lost to atresia (normal degeneration) (1).

Is it only about lower egg quantity? Or is egg quality affected too?
During a women’s 20s and early 30s, there is about a 1 in 4 chance of conceiving during any single menstrual cycle (2). However, when a woman reaches 32 years, her fertility begins to decline, with a more rapid decline from the age of 37 (3). By age 40, a woman has only about 3% of her pre-birth egg supply (3), and there is only about a 1 in 10 chance, or less, of conceiving in any single menstrual cycle (2).

But it is not just the number of eggs that matter. The eggs in an older woman have been around since before she was born, so there is a much higher risk of chromosomal abnormalities, such as Down syndrome (3).

Are there things that increase egg loss?
Yes, smoking, as well as certain chemotherapies and radiation can accelerate egg loss in females.

What other factors influence female fertility?
Older women also have a much higher risk of other disorders that can affect the chance of getting pregnant, such as uterine fibroids (growths in the uterus) and endometriosis (a painful disorder where tissue that normally lines the uterus grows outside the uterus) (3).

As women age, they also have an increased risk of complications during pregnancy, including preeclampsia, premature birth, miscarriage, and stillbirth (2).

Do sperm quantity and quality decrease with age too?
Fully understanding the influence of male age on fertility is difficult, as an older male is more likely to be having a child with an older female, so it is difficult to distinguish between the influence of the increasing female age versus the increasing male age. However, it has now been shown that fertility in males actually does also decrease with age, just not to the same extent as in females. Older males may have decreased semen quality and also have increased rates of DNA fragmentation, which can cause fertility issues. In addition, there is a link between male age and the likelihood of birth defects and chromosomal abnormalities (1).

What tests are available to determine my egg count?
A measurement of follicle-stimulating hormone (FSH) levels provides an indirect analysis of egg count or ovarian reserve as it is commonly known. We offer an FSH test, as well as a Fertility Panel that includes five biomarkers to gain a better understanding of your hormone levels and how they could be influencing your likelihood of conceiving.

1. Harris ID et al. (2011) Fertility and the Aging Male. Rev Urol. 13(4): e184-e190.
2. Having a baby after age 35: How aging affects fertility and pregnancy. ACOG. Reviewed October 2020.
3. Lewis R (Reviewed by Kallen A) (2020) How many eggs are women born with? And other questions about egg supply. Healthline.