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You asked: I’ve heard age 35 referred to as a “fertility cliff” for women. Is this real? What’s the true age of fertility decline?

Dr. Klein answers:

While 35 is an easy shorthand for the age of fertility decline, the true answer is a little more complex. Women are most fertile in their teens and early 20s, when they have about a 25% chance of getting pregnant naturally each month. Fertility declines gradually throughout your adult life—even in your 20s—and by 30, the chance of natural pregnancy is about 20% per month.

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But age 35 is not without significance. Fertility declines with age due to two factors: a decrease in egg count, as women lose eggs each month; and a decrease in egg quality, as naturally, with age, their eggs are more likely to contain chromosomal abnormalities (mistakes in their DNA). The decline in egg count and quality is much steeper in a woman’s late 30s. That’s why the impact of age-related fertility decline is strongest after age 35.

Due to a higher percentage of genetically abnormal eggs, other risks increase with age as well, such as the chance of miscarriage or Down syndrome. In a study that looked at over 1 million pregnancies, women over 35 had a much higher rate of pregnancy loss; the risk of miscarriage for women over 40 was more than 50%, and by age 45, over 90% of pregnancies ended in miscarriage. Rates of Down syndrome increase significantly as well, from 1 in 1,200 for mothers aged 25 to 1 in 30 for mothers aged 45. So while 35 is not the single “age of fertility decline,” it makes sense as a convenient shorthand.

Age-related fertility decline is a universal and consistent biological trait for all women (and for men too, though the timeline is longer). Unfortunately, the age of fertility decline cannot be extended with supplement, diets, exercise, or other therapies (though fertility decline can be steepened by certain lifestyle factors, such as smoking, or exposure to chemotherapy or other medications). In other words, women can’t extend their fertility through lifestyle changes or good habits that are beneficial to other areas of health (such as heart health, for example). That’s why egg freezing is such a powerful tool for women who want to preserve the option to have children later in life.

It’s important that women, especially women considering egg freezing, understand that fertility does not begin declining at age 35. I emphasize this because studies of fertility understanding continuously conclude that most people don’t know the age of fertility decline. For example, in a 2016 survey of 1,000 young men and women, more than 80% of all respondents believe women’s fertility only begins to decline after they turn 35.

This lack of understanding can leave women with fewer options. Because eggs are healthier and more plentiful when a woman is younger, the best age to consider egg freezing is between 27 and 34, before age-related fertility decline has a significant impact. At that point, your fertility will still be highly intact, and what you’re able to preserve is highly likely to work for you in the future. According to a study our research team at CFRE recently presented, women under 35 have a 60% chance of freezing enough eggs in one cycle for a high chance at pregnancy later; that drops to 40% for women 35–37 and 9% for women 38–40.

Despite these statistics, many women don’t begin to explore the option of egg freezing until their late 30s, because they’re not aware of the age of fertility decline (maybe because they’ve never discussed fertility with their OB/GYN). While it can be tricky to predict exactly where you’ll be in your life at age 27—and you may not be thinking about babies at all!—we urge women to consider freezing their eggs sooner, rather than later, to truly give themselves the best possible chance at success.

Having a fertility assessment early on (in your late 20s, for example) can also alert you to fertility issues, such as diminished ovarian reserve (abnormally low egg count), before it’s too late—information that can be invaluable to you as you plan for the future.

If you’re interested in a fertility assessment, or more information about the age of fertility decline, start here.