Keeping up with your biological clock: how does age matter?
Have you been constantly reminded about how your biological clock is ticking every time you blow out your birthday candles? You are not alone.
Fertility keeps changing with age for both males and females. However, for women that age is much earlier than men. For a woman, fertility is marked by the menarche (when you get your first period), and from there there is a decline, gradual at first and exponential later, till she hits the onset of menopause. The decline in fertility changes from one female to the next, but the fact remains that natural fertility in women decreases due to age-related factors.
Menstruation and Ovulation
A woman is born with all the eggs she will ever produce in her lifetime. The eggs mature in fluid filled sacs called follicles. When she starts menstruating, every month the pituitary gland located in the brain produces a hormone called Follicle Stimulating Hormone, which stimulates a group of follicles to grow in the ovaries. Usually, only one of the follicles releases a mature egg, and the rest would slowly degenerate. This is called ovulation. If the egg is fertilized by a sperm and is implanted in the inner lining of the uterus, pregnancy occurs. If the egg is not fertilised, it is shed along with the uterine lining called endometrium , resulting in blood flow. And the cycle begins again.
At birth, a woman will have about 1 million follicles. When she hits the reproductive age, the number decreases to about 300,000. Of this, not all follicles release an egg, only a few hundred end up releasing a mature egg during the reproductive years. The rest are lost in a process called atresia, regardless of how regular you are on your menses, pregnant or on birth control.
How Fertility changes with Age
As a woman ages, her fertility starts to decrease due to a decrease in the quantity as well as the quality of her eggs. She is most fertile during her 20s, when the changes of getting pregnant are about 25% each month. Fertility slowly declines in her early 30s, and particularly after 35, fertility starts to decrease rapidly. In her 30s, a healthy and fertile woman has about 20% chance of getting pregnant each month. By the time she is in her 40s, it reduces to about 5% per cycle. While we know that a woman cannot bear a child after menopause, fertility loss happens about 5-10 years before she hits menopause.
Source: Óscar Oviedo Moreno MD (gynecologist), Zaira Salvador BSc, MSc (embryologist) and Sandra Fernández BA, MA (fertility counselor), 2018, “Female Fertility – Parts of the Female Reproductive System”, https://www.invitra.com/en/female-fertility/
As a woman ages, her egg-containing follicles also decrease in number. This decrease is also termed as “loss of ovarian reserve”. As the ovarian reserve decreases, the remaining follicles are less sensitive to the Follicle Stimulating Hormone, and hence the eggs take more stimulation to mature. Eventually the follicles stop responding to the stimulus, which leads to irregular periods. The decrease in ovarian reserve is mostly age-related. But in some women, it could be due to smoking, ovarian surgery or family history of premature menopause.
As the number of eggs decrease with age, the quality of the eggs also decreases. An embryo should normally have 46 chromosomes (23 chromosomes from the egg and 23 chromosomes from the sperm). As age increases, more eggs have either too many or too few chromosomes. Hence, if and when fertilization occurs, the resulting embryo may have either too few or too many chromosomes. In most cases, either it may not result in a pregnancy at all or it could result in a miscarriage. The decrease in quality is observed mostly as a woman reaches mid to late 30s. This explains the lower rates of pregnancy and higher rates of miscarriages in women aged above 35. Between 35 and 40 years old, the chances of miscarriage are about 25-30%. This increases to more than 50% for age more than 40 years, and after 45 years the chances of miscarriage are greater than 90%.
As age is the single most important factor in determining your fertility, it is imperative for you to be proactive about your fertility and understand how your body and lifestyle impact the health of your fertility hormones. Being aware of all the options available at each point in time will help you access the most suitable options and take the best decisions to achieve your needs and goals.