It’s a cruel irony that when it’s biologically easiest for a woman to get pregnant and have a baby, many women aren’t remotely interested in doing so. And yet many women don’t realise the extent to which time isn’t in their favour on this front: In fact, a U.S. survey of a thousand women between the ages of 18 and 40 found that 20 percent were unaware of the effects of age on their fertility.
“The average age when women are looking to get pregnant has been steadily increasing,” says Tarun Jain, a reproductive endocrinologist and medical director of the Centre for Fertility and Reproductive Medicine at Northwestern Medicine in Chicago. This is often because when their fertility is at its peak, women are focused on their careers and/or not ready to settle down with the right partner.
“Many women are really surprised to learn how much fertility declines with age,” Jain says. As women get older, he adds, “fertility goes down, miscarriage rates go up, and birth defects go up. It’s a more painful journey the older a woman gets.”
The reality is: A woman’s fertility (a.k.a., the ability to get pregnant) is at its peak between the late teens and late 20s, according to the American College of Obstetricians and Gynaecologists (ACOG). By 30, a woman’s fertility starts to decline, and after 35 the rate accelerates. Specifically, before 30, women have an 85 percent chance of conceiving within a year; at 30, those odds drop to 75 percent; and at 35 the chance drops to 66 percent, research has found. At 40, a woman has only a 44 percent chance of conceiving within 12 months.
“Fertility lies on a continuum,” says Sandra Ann Carson, a reproductive endocrinologist and obstetrician-gynaecologist at Yale University, explaining that it’s a gradual descending slope.
What causes the fertility drop-off
Unlike a man, who can manufacture a new batch of sperm every 72 days, a woman is born with all the eggs she is ever going to have—approximately one to two million in her ovaries. This amount declines to between 300,000 and 500,000 at puberty and continues its downward trajectory from there. During the years that a woman is menstruating, she loses eggs every month. “Many women think they’re losing one egg at a time but it’s a cohort of 10 to 20 per month,” says Jain.
When a mature egg is released from the ovary during ovulation, there’s a 12- to 24-hour window when it can be fertilised by sperm. “The other eggs die off through a process called apoptosis,” Jain explains. (Apoptosis is a biologically programmed cell death that’s an inherent part of human ovarian function.) By the time a woman reaches 37, her egg count is down to 25,000 and at 51, which is the average age of menopause in the U.S., she has at most a thousand eggs left in her ovaries.
“It’s part of ageing—as we get older, we get wrinkles and our metabolism slows down, and we continue to lose eggs,” says R. Kate Byron, an obstetrician-gynaecologist at the Ohio State University in Columbus.
It’s not just a numbers issue, however. It’s a matter of quality, too, because by age 45 or 50, most of the eggs that remain in the ovaries are chromosomally abnormal, says Joseph Hill, a reproductive endocrinologist and fertility specialist at Fertility Centres of New England which has locations in Massachusetts, New Hampshire, and Maine. “Most chromosomally abnormal eggs don’t allow fertilisation. Of those that do, most don’t develop into embryos that could implant [in the uterus]. Of those that do implant, 70 percent are lost in the first 11 weeks through miscarriage.”
Indeed, the number of chromosomally normal eggs a woman has declines with age, notes Carson. As you get older, you have a higher number of genetically abnormal eggs, and each month after ovulation a higher percentage of abnormal eggs are left in reserve.
By contrast, men don’t experience an age-related hit to their fertility: It’s true that sperm quality deteriorates somewhat as men get older, but a man essentially has a clean slate for sperm production every two and a half months. As the American Society for Reproductive Medicine (ASRM) says, “there is no maximum age at which a man cannot father a child.”
“It’s so unfair—as men age, they often have trouble with erections, but their fertility doesn’t change much, whereas women’s libido tends to increase but they’re less fertile,” says Carson, a former president of the ASRM.
Stealthy fertility foilers
Besides chronological age, genetic factors may influence how quickly a woman’s eggs die off. “Some women’s egg reserve declines at a faster rate than others’ do,” Jain notes. “There’s probably some kind of biologic programming tied to that.” On the other hand, he adds, “the uterus doesn’t age—it’s just the ovaries, which is why pregnancy can occur in older women using a donor egg.”
In addition, some lifestyle factors, exposure to environmental toxins including pesticides and chemicals in plastics (such as bisphenol A), and certain medical conditions can impact egg quality. “As women age, there’s more time for lifestyle factors and reproductive toxins to damage the eggs that are in reserve,” Carson says.
On the lifestyle front, smoking is toxic to eggs and damages them prematurely, which is why smokers often go through menopause earlier than non-smokers do, Hill notes. A study in a 2022 issue of the journal PLoS One found that women who are heavy smokers (more than 10 cigarettes per day) or long-term smokers have a greater risk of diminished ovarian reserve, a condition in which the quantity and quality of a woman’s eggs are lower than expected for her age. And a study in a 2016 issue of BMJ found that women, ages 21 to 45, whose alcohol consumption was heavy—on the order of 14 or more drinks per week—had an 18 percent decreased chance of getting pregnant over the course of a year.
Meanwhile, obesity may negatively affect a woman’s fertility. A study involving more than 2,000 women of reproductive age in the U.S. and Canada found that those with a body mass index (BMI) between 35 and 39 had a 22 percent lower chance of getting pregnant in any given menstrual cycle compared to women with a healthy BMI (between 18.5 and 24); for those with a BMI between 40 and 44 the odds were worse, at 39 percent; those with a BMI of 45 or higher had the lowest likelihood of conceiving, with a 58 percent decreased chance compared to those with a healthy BMI. “With excess weight, an inflammatory reaction occurs that can affect egg quality and implantation,” Hill explains.
Other factors unrelated to egg quality also can compromise fertility in women. Having a history of certain sexually transmitted infections, such as chlamydia or gonorrhoea, can cause blocked fallopian tubes or tubal scarring that can thwart fertility in women, Byron notes. That’s why it’s important to use condoms or limit the number of sexual partners, experts say.
Hormonal disorders that interfere with ovulation can compromise fertility. Polycystic ovary syndrome (PCOS)—a condition marked by higher than usual levels of male hormones called androgens, and often excess body weight and insulin resistance—can cause fertility problems. The same is true of endometriosis—in which tissue that’s similar to the lining of the uterus grows outside of it. Fibroids (benign growths of muscle and fibrous tissue that develop in the uterus) can sometimes cause problems that can increase the risk of miscarriage, Byron says. And if a woman has had chemotherapy or radiation to the pelvis as part of cancer treatment, those interventions can affect the ovaries, causing them to stop releasing eggs, Jain notes.
“Many women don’t understand that fertility is multifactorial,” Byron says. When it comes to egg health, advancing age delivers the harshest blow to a woman’s fertility, which is why it’s important to think about a pregnancy plan or timeline before age 35, Byron says. “Time is of the essence.”