More and more women are delaying having children in order to focus on their careers, becoming financially secure and finding the right person. In fact, now women in the US delay having a baby for longer than women in any other country. And with the media coverage of celebrities giving birth for the first time in their forties, it’s easy to imagine why many women believe they have all the time in the world.

However, the truth is that Female fertility is affected by age. After puberty, female fertility increases and then decreases with advancing age, causing an increased risk of female infertility. In humans, a woman’s fertility peaks in the early and mid-20s, after which it starts to decline slowly, with a more dramatic drop starting at around 35. Menopause, or the cessation of menstrual periods, generally occurs in the 40s and 50s and marks the cessation 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” and when a woman reaches an age where fertility is commonly understood to drop, it is commonly said that her “biological clock is ticking.”

Many women are unaware that female reproductive aging follows a pattern of decrease in egg quantity and quality and that they do not regenerate. Women are surprised to find out that the most eggs they will every have in her lifetime is during fetal life when the egg numbers peak at around 6 million to 7 million. From that point on a women loses eggs through a process called “atresia” where eggs are continually dying off. Approximately 1-2 million eggs are present at birth, and with continual natural loss of eggs, a women enters puberty with 300,000-500,000 eggs. That may seem like a lot of eggs, however, every month a woman loses approximately 700-1000 eggs. By the time a women reaches the age of 25, her egg number are cut in half to 130,000 and by the time she is 35 she is left with approximately 70,000 eggs. At age 37 there is a marked increase in egg loss such that by the time a women reaches age 40 she is left with only about 13,000 eggs.

With age, 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 20’s and significantly deteriorating after age 35. There are many factors that can influence the quantity and quality of eggs such as genetics, environment, or medical issues (endometriosis, ovarian surgery, chemotherapy, and radiation therapy). Lifestyle factors such as smoking can cause a rapid decrease in egg quality and quantity. In fact, women who smoke enter the 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.

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 a women’s so called reserve of eggs (ovarian reserve) is to perform a blood test on day 3 of the menstrual cycle to measure serum FSH and estrogen levels. Alternatively, a blood test to measure the serum AMH (antimullerian hormone) level can give similar information. Transvaginal ultrasound can also be used to “count the number of follicles.” This procedure is called Antral Follicle Count. It is important to recognize that a poor result from ovarian reserve testing does not signify an absolute inability to conceive.

The inverse correlation between age and female fertility in later reproductive life is a strong argument for addressing family planning well before having reached 35 years of age. Mapping of a woman’s ovarian reserve can give an individual prognosis about future chances of pregnancy, facilitating an informed choice of when to have children. Notably, a higher level of anti-Müllerian hormone 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.