Childbearing age refers to the years during which a person is biologically capable of becoming pregnant and carrying a pregnancy to term. Health agencies like the CDC define this range as 15 to 49, though fertility is far from equal across those years. A woman in her early to mid-twenties has a 25 to 30% chance of conceiving in any given month, while a 40-year-old has roughly a 5% chance.
How the Range Is Defined
The biological window for childbearing opens with the first menstrual period, which currently averages around age 12 in the United States. It closes with menopause, which occurs at an average age of 52. The CDC’s National Survey of Family Growth uses ages 15 to 49 as its standard range for studying fertility, and in 2015 expanded its upper boundary from 44 to 49 specifically to capture the experiences of women at the later end of their reproductive years.
These boundaries reflect biological possibility, not optimal timing. The years at both extremes of this range carry higher health risks for both parent and baby.
When Fertility Peaks
Fertility is highest in the early to mid-twenties. A large North American preconception study found that women aged 25 to 27 had the highest probability of becoming pregnant within 12 months of trying, at about 79%. Women in the same study who were 40 to 45 had a 55.5% probability over the same timeframe.
What’s notable is how gradual the early decline is. Compared to women aged 21 to 24, those in their late twenties and early thirties conceived at nearly identical rates. The drop didn’t become statistically meaningful until around age 37, when fertility fell by about 25%. By ages 40 to 45, the monthly chance of conception was roughly half that of a woman in her early twenties.
The Decline After 35
Fertility starts a slow decline in the early thirties, then accelerates after 35. This is the age the American College of Obstetricians and Gynecologists has historically used to define “advanced maternal age,” based on evidence of both declining fertility and rising risk of chromosomal abnormalities in offspring. By 40, the chance of becoming pregnant in any single monthly cycle drops to around 5%.
The decline reflects changes in both egg quantity and egg quality. Women are born with a fixed number of eggs, and as those eggs age, the likelihood of chromosomal errors during cell division increases. This is why conditions like Down syndrome become more common with maternal age.
Pregnancy after 35 also carries higher rates of complications. Studies show that women 35 and older face increased risk of preeclampsia, gestational diabetes, cesarean delivery, and postpartum hemorrhage. Women over 40 have two to three times the risk of gestational diabetes compared to younger women, and are two to four times more likely to have chronic high blood pressure than women aged 25 to 29.
Risks at the Younger End
The lower end of the childbearing range carries its own set of concerns. Teen pregnancies are associated with higher rates of premature birth (about 1.7 times more likely), low birth weight, and neonatal intensive care admission. One study found that perinatal mortality among teen mothers was 18.7 per 1,000 births, compared to much lower rates in older age groups, and neonatal mortality was more than five times higher than in the control population.
Some of these risks stem from the body still developing, but many are tied to socioeconomic factors: inadequate prenatal care, poor nutrition, and higher rates of smoking or substance use among younger mothers. Maternal anemia and preterm labor are the two most common complications in this group.
How Age Affects Assisted Reproduction
For those who turn to IVF, age remains the single strongest predictor of success. After one IVF cycle, women 35 and under have about a 37% chance of a live birth. That figure rises to nearly 70% after multiple cycles. Women aged 36 to 39 see similar cumulative results, reaching about 84% after six cycles.
The picture changes dramatically after 40. A single IVF cycle at age 40 or older yields a live birth only about 3% of the time. Even after six cycles, the cumulative rate reaches just 31%. Australian data tells a similar story: live birth rates per complete IVF cycle run about 43% for women 30 to 34, 31% for women 35 to 39, and 11% for women 40 to 44.
Paternal Age Matters Too
Conversations about childbearing age tend to focus on women, but male fertility also declines with age. Sperm quality, including motility, shape, and viability, begins to deteriorate noticeably after age 35. After 40, sperm count and the percentage of viable sperm both drop. Men older than 34 take longer to achieve pregnancy with a partner, and the odds of successful fertility treatments decrease as men age.
The effects extend beyond conception. A father’s age increases the chance of new genetic mutations in offspring by about 4% per year. Men over 45 face a 48% higher risk of stillbirth compared to men aged 25 to 29. Miscarriage risk also climbs: even when the mother’s age is held constant at 20 to 29, the odds of miscarriage nearly double when the father is 40 or older compared to when he is under 35. Children of older fathers also show higher rates of certain conditions, including some psychiatric and developmental disorders.
The Transition to Menopause
Fertility doesn’t end abruptly. Perimenopause, the transition phase leading to menopause, typically begins in the mid-to-late forties and lasts about four years on average, though it can stretch anywhere from two to eight years. During this time, ovulation becomes irregular and unpredictable. Pregnancy is still possible during perimenopause, but increasingly unlikely. Menopause itself, defined as 12 consecutive months without a period, arrives at an average age of 52 in the United States, marking the biological end of the childbearing years.