For many women thinking about starting a family, one number tends to loom larger than any other, ‘35’.

It is the age when pregnancies are often labeled as being of "advanced maternal age", a term that replaced the older, more alarming phrase "geriatric pregnancy." Over the years, the message has become ingrained, once you turn 35, getting pregnant suddenly becomes much more difficult.

But fertility experts say that belief oversimplifies what actually happens.

"There isn't a dramatic biological cliff at age 35," said Emily Mann, a sociologist at the University of South Carolina in her research, Journal of Health and Social Behavior. She notes that the idea has become embedded in public thinking, despite fertility being far more nuanced than a single age threshold suggests.

Recent birth trends shows that change. According to Centers for Disease Control and Prevention data released in 2025, while the overall fertility rate declined in 2024, birth rates among women aged 35 to 39 remained stable, and births among women over 40 increased by 2 percent.

Experts say changing life priorities partly explain the trend. Many women are delaying parenthood to complete higher education, achieve financial stability, establish careers, or find the right partner. Advances in assisted reproductive technologies, including in vitro fertilization (IVF), have also made later pregnancies more common than they once were.

Still, doctors stress that age remains the single biggest factor influencing fertility.

"That's incontrovertible," said Kara Goldman, MD, associate professor of obstetrics and gynecology at Northwestern University in her research, Fertility and Sterility. 

For women younger than 35, infertility is generally defined as not becoming pregnant after trying for one year. But while fertility gradually declines with age, it does not suddenly collapse the day someone turns 35. Individual health, genetics, lifestyle and even a partner's fertility all play important roles.

Why 35 became such an important number

Years ago, doctors observed that around age 35, the chance of carrying a fetus with a chromosomal abnormality became roughly equal to the risk of miscarriage from amniocentesis, an invasive prenatal diagnostic test. That balance made 35 an important clinical marker, and over time it became widely associated with higher-risk pregnancies.

The number simply "stuck," even though fertility changes occur gradually rather than overnight.

Risks linked to pregnancy including miscarriage and chromosomal abnormalities, do rise with age, and those risks increase more rapidly during the late 30s than they do during the 20s, according to Allen Willcox, Epidemiologist in BMJ research.

The odds of conception also changes over time. Healthy couples in their 20s and early 30s have roughly a 25 to 30 percent chance of conceiving during any single menstrual cycle. By age 40, that chance falls to below 10 percent per cycle.

What changes as women get older?

One of the biggest factors is the natural decline in egg reserve.

Women are born with all the eggs they will ever have around one to two million. That number steadily decreases throughout life, and by age 37, only about 25,000 remain.

The speed of that decline, however, differs from person to person. Smoking can accelerate egg loss, while conditions such as obesity and diabetes may also affect ovarian reserve.

Researchers are also exploring the role of genetics. Although a woman whose mother conceived naturally at 40 is not guaranteed to have the same experience, family history may offer some clues about reproductive lifespan.

Egg quality also changes with age. As women get older, a greater proportion of their remaining eggs develop chromosomal abnormalities, making conception more difficult and increasing the likelihood of miscarriage.

Hormones contribute as well. Over time, the ovaries gradually produce lower levels of estrogen and progesterone, affecting ovulation and eventually leading to menopause.

Age also raises the likelihood of health conditions that may interfere with pregnancy. Fibroids, which commonly affect women between 30 and 50, can contribute to infertility. The risk of developing diabetes, obesity and autoimmune diseases, all associated with reduced fertility also increases with age.

Men aren't immune to age-related fertility changes

Although male fertility has historically received less scientific attention, experts say age matters for men too.

Unlike women, men continue producing sperm throughout life. However, testosterone levels gradually decline after the 40s, and sperm quality also worsens. Older sperm may move less efficiently and show subtle structural changes that make fertilization more difficult.

Chronic illnesses such as diabetes and high blood pressure can further affect fertility, and researchers are also investigating lifestyle factors including marijuana use that may contribute to declining reproductive health.

Does turning 35 change your fertility plan?

Current guidelines recommend that women over 35 seek an infertility evaluation after trying to conceive for six months without success, compared with one year for younger women. The recommendation is designed to avoid losing valuable time if treatment is needed.

Ultimately, fertility does decline with age, but it is a gradual process influenced by many factors, not a sudden drop triggered by a birthday.