A woman’s fertility starts declining gradually in her early 30s and drops more sharply after 37. In her early to mid-20s, a woman has roughly a 25 to 30 percent chance of conceiving in any given month. By 40, that monthly chance falls to about 5 percent. The decline comes down to two things happening simultaneously: fewer eggs remaining and a higher proportion of those eggs carrying chromosomal errors.
The Egg Supply Is Fixed From Birth
Women are born with all the eggs they will ever have, roughly 2 million. There is no way to produce new ones. By puberty, that number has already dropped to about 400,000. The loss continues steadily through adulthood, and by age 37, approximately 25,000 eggs remain. That sounds like a lot, but the rate of loss accelerates from this point, and most of those eggs will never mature into ones that can be fertilized.
This fixed, shrinking supply is called ovarian reserve. Doctors can estimate it with a blood test that measures a hormone produced by developing follicles in the ovaries. Levels below a certain threshold in younger women suggest the reserve is declining faster than expected. A separate blood test, taken early in the menstrual cycle, measures the hormone that stimulates your ovaries to prepare an egg each month. When that hormone rises above normal levels, it signals that the brain is working harder to coax a response from a smaller pool of follicles.
Egg Quality Matters More Than Quantity
The number of eggs is only half the story. The quality of those eggs, specifically whether they contain the correct number of chromosomes, changes dramatically with age. In women younger than 35, about 30 percent of embryos tested during fertility treatment are chromosomally abnormal. In women older than 44, that figure exceeds 90 percent.
Chromosomal errors happen during cell division as an egg matures. The machinery that separates chromosomes becomes less reliable over time, so older eggs are far more likely to end up with too many or too few chromosomes. An embryo with the wrong number typically either fails to implant, miscarries in the first trimester, or, in rarer cases, results in a chromosomal condition like Down syndrome. This is the single biggest reason fertility drops so steeply in the late 30s and 40s: even when conception occurs, the pregnancy is less likely to be viable.
Monthly Odds of Conception by Age
The practical impact of these biological changes shows up clearly in conception rates. A healthy woman in her early to mid-20s who is trying to conceive has a 25 to 30 percent chance of success each menstrual cycle. That per-cycle probability holds fairly steady through the late 20s, then begins a gradual decline through the early 30s.
By the late 30s, the monthly odds have dropped noticeably. By 40, the chance of conceiving naturally in any given cycle is around 5 percent. That means a 40-year-old woman trying to conceive could expect, on average, to need about 20 months of trying for a single positive pregnancy test, and even then the pregnancy carries a higher risk of loss.
Miscarriage Risk Rises Steeply After 35
Because chromosomal errors become more common with age, miscarriage rates climb in a pattern that mirrors egg quality decline. Between ages 20 and 30, the chance of miscarriage is roughly 9 to 17 percent. At 35, it rises to about 20 percent, or 1 in 5 pregnancies. At 40, the risk doubles to 40 percent. By 45, approximately 80 percent of pregnancies end in miscarriage.
These numbers can feel alarming, but they help explain a common experience: women in their late 30s and 40s who conceive relatively quickly but then face repeated early losses. The issue is rarely the ability to get pregnant at all. It is the ability to get pregnant with a chromosomally normal embryo that can develop into a healthy baby. Each loss represents an egg that carried errors incompatible with a continuing pregnancy.
Pregnancy Complications After 35
Age affects more than just getting pregnant. It also influences what happens during pregnancy. Conditions like gestational diabetes, preeclampsia (dangerously high blood pressure during pregnancy), cesarean delivery, and postpartum hemorrhage all become more common as maternal age increases. Research from the American College of Obstetricians and Gynecologists shows the risk of serious pregnancy complications rises in a stepwise fashion: moderately higher for women 35 to 39, nearly twice as high for women 40 to 44, and more than three times as high for women 45 to 54, compared to younger women.
Stillbirth risk also increases with age. Current obstetric guidelines recommend additional fetal monitoring during the third trimester for women who will be 40 or older at delivery. This typically means more frequent ultrasounds or other assessments in the final weeks to check that the baby is thriving.
IVF Does Not Fully Overcome Age
Many women assume that fertility treatment, particularly IVF, can compensate for age-related decline. It helps, but it cannot reverse the underlying biology. IVF success rates follow the same age curve as natural conception because the treatment still depends on the quality of a woman’s own eggs.
Data from the Society for Assisted Reproductive Technology show that live birth rates per embryo transfer are highest for women under 35 and decline steadily from there. By the early 40s, multiple IVF cycles may be needed to achieve a single live birth, and many women in this age group will not succeed with their own eggs at all. Using eggs from a younger donor eliminates the age-related quality problem, which is why donor egg IVF maintains high success rates regardless of the recipient’s age.
When to Seek Help
The American Society for Reproductive Medicine recommends that women under 35 try to conceive for 12 months before seeking a fertility evaluation. For women 35 and older, that timeline shortens to 6 months, reflecting the faster pace of decline and the benefit of earlier intervention. Women over 40 may warrant more immediate evaluation and treatment, even before the six-month mark, because each passing cycle carries a meaningful cost in terms of remaining fertility.
If you are in your early 30s and not yet ready to try for a baby, egg freezing preserves eggs at their current quality. Freezing eggs at 32 captures that 30 percent chromosomal error rate rather than the much higher rates you would face at 40 or beyond. The younger the eggs at the time of freezing, the better the odds when they are eventually used.