Tick Tock: The Biological Clock Isn’t a Myth
It’s become increasingly common for women to put off having children in order to focus on their careers, become financially secure and find the right person. So much so that now women in the U.S. delay having a baby for longer than women in any other country. And with media coverage of celebrities giving birth for the first time in their 40s, it’s easy to see why many women believe they have all the time in the world.
Female Fertility Affected By Age
Despite what we see in the press, the truth is that female fertility is, without a doubt, affected by age. After puberty, female fertility increases and then decreases, with advancing age causing an increased risk of female infertility. A woman’s fertility generally follows this timeline:
- Peaks in the early and mid-20s, after which it starts to decline slowly.
- Drops more dramatically around 35 years of age.
- The cessation of menstrual periods, or menopause, generally occurs in the 40s and 50s.
- Menopause marks the end of fertility, although age-related infertility can occur before then.
The relationship between age and female fertility is popularly referred to as a woman’s “biological clock” when a woman reaches an age where fertility is commonly understood to drop, it can be said that her biological clock is ticking.
Egg Quantity & Quality
Many women are unaware that female reproductive aging follows the generally accepted theory that over time eggs decrease in quantity and quality and do not regenerate. Women are surprised to find out that the most eggs she will ever have in her lifetime is during fetal life when the egg numbers peak at around 6-7 million. From that point on a woman loses eggs through a natural process called “atresia” where eggs continually dye off. After puberty, a woman loses approximately 700-1,000 eggs per month.
Egg Quantity Throughout a Woman’s Life
- Birth: 1-2 million eggs
- Puberty: 300,000 – 500,000 eggs
- Age 25: 130,000 eggs
- Age 35: 70,000 eggs
- Age 40: 13,000 eggs
The eggs not only begin to diminish in quantity, but also in quality. The combination of these factors leads to a woman’s fertility beginning to decline in her 20s and significantly deteriorating after age 35.
Factors That Influence Egg Quantity & Quality
Many things can influence the quantity and quality of eggs such as:
- Medical issues (endometriosis, ovarian surgery, chemotherapy and radiation therapy)
- Smoking causes a rapid decrease in egg quality and quantity. In fact, women who smoke enter menopause at an earlier age than women who don’t smoke.
Although this reproductive decline occurs with age, there is a significant variation in fertility among women of similar age, which highlights the unpredictability and individuality of the reproductive aging process.
Determining Your Reproductive Health
Each woman is different when it comes to fertility and we can now use blood tests and ultrasound scans to see the fertility potential by looking at egg reserves. The more egg reserves you have the better your fertility, however this won’t tell us about the quality of the eggs.
The most common methods of checking the status of a woman’s reserve of eggs (ovarian reserve) are:
- FSH: A blood test is performed on day 3 of the menstrual cycle to measure serum FSH (follicle stimulating hormone) and estrogen levels.
- AMH: Alternatively, a blood test to measure the serum AMH (antimullerian hormone) level can give similar information.
- Antral Follicle Count: A transvaginal ultrasound can also be used to “count the number of follicles.”
It’s important to recognize that a poor result from ovarian reserve testing does not signify an absolute inability to conceive. Also, notably, a higher level of antimullerian hormone, when tested in women in the general population, has been found to have a positive correlation with natural fertility in women aged 30-44 aiming to conceive spontaneously, even after adjusting for age. Thus, AMH measurement is helpful to determine which women may need to conceive at an earlier age, and which women can potentially wait.
Why the Biological Clock Matters
The inverse correlation between age and female fertility in later reproductive life is argued to motivate family planning well before 35 years of age. Mapping a woman’s ovarian reserve can give an individual a good sense of her future chance of pregnancy, facilitating an informed choice of when to have children.