Women are most fertile in their early to mid-20s, with conception rates remaining strong through the early 30s before declining more noticeably after 35. In a large North American study, about 79% of women aged 25 to 27 conceived within 12 months of trying, the highest rate of any age group tracked. But fertility doesn’t fall off a cliff at 30, and the real numbers are more nuanced than most people expect.
Fertility by Age: The Actual Numbers
The clearest way to understand fertility across age is to look at how many women conceive within a year of trying. A preconception cohort study published in the American Journal of Obstetrics and Gynecology tracked thousands of women and found the following 12-month conception rates:
- Ages 21 to 24: 71%
- Ages 25 to 27: 79%
- Ages 28 to 30: 78%
- Ages 31 to 33: 77%
- Ages 34 to 36: 75%
- Ages 37 to 39: 67%
- Ages 40 to 45: 56%
A few things stand out. The 21-to-24 group actually had a lower conception rate than the 25-to-30 group, likely because older women in the study were more intentional about timing intercourse and tracking ovulation. Biologically, though, the per-cycle chance of conception (fecundability) was highest in the youngest group and declined steadily from there. Compared to women aged 21 to 24, women aged 34 to 36 had about 18% lower odds of conceiving in any given cycle. By 37 to 39, that gap widened to 40% lower odds per cycle. And by 40 to 45, per-cycle fertility dropped to roughly 40% of what it was in the early 20s.
The practical takeaway: fertility in the late 20s and early 30s is still very high. The decline is real but gradual until the mid-30s, when it picks up speed.
Why Fertility Declines With Age
Women are born with all the eggs they will ever have. A female fetus at 20 weeks of gestation carries around 6 to 7 million egg cells. By birth, that number has already dropped to 1 to 2 million. At puberty, roughly 300,000 to 500,000 remain. By age 37, only about 25,000 are left, and by age 51, the average is around 1,000.
But it’s not just the number of eggs that matters. Egg quality also drops with age, and this is the bigger factor in declining fertility. As eggs age, they become more prone to chromosomal errors during cell division. These errors mean the egg has the wrong number of chromosomes, which usually prevents a viable pregnancy or leads to early miscarriage. Before age 35, chromosomal abnormalities in eggs occur at low rates, generally under 5% for most types. After 35, those rates climb steadily. After 38, there is a roughly 10-fold increase in errors involving multiple chromosomes at once.
The underlying causes include accumulated DNA damage over time, deterioration of the protein structures that hold chromosomes together during cell division, shortening of protective caps on chromosomes (telomeres), and declining function of the energy-producing components within each egg cell. These aren’t things you can feel or prevent through lifestyle changes alone. They are a built-in feature of how human eggs age.
The Key Acceleration Points: 32, 35, and 37
Fertility doesn’t decline at a constant rate. There are specific ages where the pace of decline shifts. The egg supply decreases steadily until about age 32, then starts dropping faster. At 37, the rate of loss accelerates again. This is why 37 is sometimes called a biological inflection point for fertility.
Age 35 is often cited as a clinical threshold, but it’s not a magic number where fertility suddenly collapses. It’s more accurate to think of the mid-30s as the transition zone where both egg quantity and quality are declining fast enough to meaningfully affect your monthly odds of conceiving. A 34-year-old and a 36-year-old won’t typically notice a dramatic difference. But the gap between 34 and 39 is significant.
Miscarriage Risk Rises Alongside Fertility Decline
Declining egg quality doesn’t only make it harder to conceive. It also increases the chance that a pregnancy will end in miscarriage, because embryos with chromosomal abnormalities often don’t survive the first trimester. A large population-based study in Denmark found these miscarriage rates by age:
- Ages 20 to 24: about 11%
- Ages 25 to 29: about 12%
- Ages 30 to 34: about 15%
- Ages 35 to 39: about 25%
- Ages 40 to 44: about 51%
- Age 45 and older: about 93%
The jump between the mid-30s and early 40s is stark. By 40, roughly half of recognized pregnancies end in miscarriage. This means that even when conception happens, the likelihood of carrying to term drops considerably.
Fertility After 40
Natural conception after 40 is possible but significantly less likely. The North American cohort study found that about 56% of women aged 40 to 45 conceived within a year, but that number came with a very wide confidence range (31% to 80%), reflecting huge variability between individuals. Per-cycle odds are low, and the combined effect of fewer eggs, more chromosomal errors, and higher miscarriage rates means the path from conception to live birth narrows considerably.
Data from exceptionally fertile populations puts the natural birth rate for women over 45 at roughly 0.2% to 0.4% of total deliveries. A Chinese study found that only 1.8% of women aged 41 to 49 achieved a natural pregnancy. Assisted reproductive technology can improve the odds, particularly when using eggs from a younger donor, but success rates with a woman’s own eggs decline sharply after 42 or 43.
Getting a Snapshot of Your Own Fertility
Because every woman’s egg reserve declines at a slightly different rate, blood tests can give you a personalized picture. The most common is the AMH (anti-Müllerian hormone) test, which estimates how many eggs you have remaining. Typical AMH levels by age fall along these lines:
- Age 25: about 3.0 ng/mL
- Age 30: about 2.5 ng/mL
- Age 35: about 1.5 ng/mL
- Age 40: about 1.0 ng/mL
- Age 45: about 0.5 ng/mL
These figures represent the lower end of average for each age. An AMH below 1.0 ng/mL is considered low, and below 0.4 ng/mL is severely low. AMH tells you about egg quantity, not quality, so it’s one piece of the puzzle rather than the whole picture. Still, it can be useful if you’re planning to delay pregnancy and want to know where you stand relative to your age group.
Timeline Recommendations for Trying to Conceive
The American Society for Reproductive Medicine recommends different timelines for seeking a fertility evaluation depending on your age. If you’re 35 or younger and have been trying for 12 months without success, it’s time for an evaluation. If you’re 36 to 40, that window shortens to 6 months. If you’re over 40, the recommendation is to seek evaluation right away rather than waiting. These aren’t arbitrary cutoffs. They reflect the reality that time matters more as egg reserves shrink, and earlier intervention generally leads to better outcomes.
For women who know they want children but aren’t ready yet, a consultation with a reproductive specialist is worth considering even before you start trying. Understanding your AMH level and overall reproductive health can help you make more informed decisions about timing, and about whether egg freezing or other options make sense for your situation.