Low fertility means your body can conceive, but the chances of getting pregnant in any given cycle are lower than average. It’s not the same as infertility, which is clinically defined as the failure to achieve pregnancy after 12 months of regular unprotected sex (or 6 months if you’re 35 or older). About 1 in 6 adults worldwide, roughly 17.5%, experience some form of fertility difficulty, and the rates are similar across high-income and low-income countries.
Low fertility sits on a spectrum. Some people conceive quickly, some take longer, and some need medical help. Understanding where you fall on that spectrum depends on several measurable factors.
How Conception Odds Change With Age
Age is the single strongest predictor of natural fertility, particularly for women. A large North American study tracking couples actively trying to conceive found that women aged 25 to 27 had a 79.3% chance of becoming pregnant within 12 cycles. By ages 34 to 36, that dropped slightly to 74.8%. But by ages 40 to 45, the 12-cycle pregnancy rate fell to 55.5%. The steepest drop happens after 35, and it accelerates through the early 40s.
These numbers represent averages across healthy couples with no known fertility issues. If your personal odds are meaningfully below these benchmarks for your age group, that’s what doctors mean by low or reduced fertility.
Common Causes in Women
Female factors contribute to roughly half of all fertility difficulties. The causes fall into a few major categories.
Ovulation problems are the most common, responsible for 25% to 40% of female infertility cases. Polycystic ovary syndrome (PCOS) drives about 70% of ovulation-related issues. In PCOS, hormonal imbalances and insulin resistance interfere with egg development, so eggs either don’t mature properly or aren’t released on a regular schedule. Other hormonal disruptions, like those caused by high prolactin levels or extreme stress and weight loss suppressing reproductive hormones, can also prevent ovulation entirely.
Fallopian tube damage accounts for 20% to 35% of cases. Infections, prior pelvic surgery, or endometriosis can cause scarring that blocks the tubes or impairs their ability to transport an egg. Endometriosis in particular creates chronic inflammation and adhesions that distort the pelvic anatomy.
Uterine factors make up another 10% to 15%. Fibroids that press into the uterine cavity can reduce blood flow and make it harder for an embryo to implant. Scar tissue inside the uterus (Asherman syndrome) can reduce pregnancy rates by approximately 70%. Structural differences present from birth, like a wall of tissue dividing the uterine cavity, raise miscarriage risk because of poor blood supply at the implantation site.
Common Causes in Men
Male factors are involved in roughly half of couples struggling to conceive. A semen analysis measures several key parameters: sperm concentration, motility (how well sperm swim), and morphology (sperm shape). The WHO sets lower reference limits based on the bottom 5th percentile of men whose partners conceived within a year. The current thresholds are a sperm concentration of at least 16 million per milliliter, total motility of 42% or higher, and normal morphology of at least 4%.
Falling below any one of these thresholds can compromise natural conception. However, semen analysis results exist on a continuum. A man with slightly low motility still has a reasonable chance of conceiving naturally, while someone with severely low counts or no moving sperm faces a much steeper challenge. The test can’t precisely separate fertile from infertile men except in extreme cases like a complete absence of sperm.
How Low Fertility Is Measured in Women
Two tests give the clearest picture of a woman’s ovarian reserve, which is essentially how many eggs remain available.
Anti-Müllerian hormone (AMH) is a blood test that reflects the size of your remaining egg pool. Average levels fall between 1.0 and 3.0 ng/mL. Below 1.0 ng/mL is considered low, and below 0.4 ng/mL is severely low. These numbers naturally decline with age: a typical 30-year-old might have an AMH around 2.5 ng/mL, while a 40-year-old might sit around 1.0 ng/mL. A result significantly below the expected value for your age suggests diminished ovarian reserve.
An antral follicle count (AFC) uses transvaginal ultrasound to count the small follicles visible on each ovary early in your menstrual cycle. A count of 3 to 7 signals a meaningful decline in ovarian reserve. A cutoff of about 5 to 6 total follicles distinguishes poor ovarian response from normal with roughly 90% accuracy. When both AMH and AFC are low, the probability of a live birth through IVF drops to around 5.6%.
The Role of Body Weight
Body weight affects fertility in both directions. A study of nearly 15,000 IVF patients found that cumulative live birth rates stayed relatively stable across BMIs from 18.5 to about 30. Below 18.5 (underweight), outcomes trended lower, though the difference wasn’t statistically significant after adjusting for other factors. Above a BMI of 30, the picture changes sharply: obese patients saw about a 40% reduction in their odds of a live birth compared to normal-weight patients. For every one-unit increase in BMI above 30, live birth rates dropped by roughly 12%.
The mechanism works both ways. Excess body fat increases estrogen production and insulin resistance, which can disrupt ovulation. Too little body fat can suppress the hormonal signals that trigger egg development altogether. For men, obesity is linked to lower sperm concentration, reduced testosterone, and higher rates of DNA damage in sperm.
What Low Fertility Means for Treatment
Low fertility doesn’t mean you can’t conceive. It means the probability per cycle is reduced, and you may need more time or medical assistance. Treatment depends entirely on the underlying cause. Ovulation problems are often the most responsive to treatment, since medications can stimulate egg development directly. Tubal or uterine issues may require surgical correction or IVF to bypass the problem entirely.
For women with diminished ovarian reserve, age matters more than the reserve itself when predicting IVF success. Women under 35 with very low AMH levels still achieved a clinical pregnancy rate of 31% per egg retrieval. Between 35 and 39, that rate was 23.2%. After 40, it dropped to 10.2%. The quality of remaining eggs, which correlates strongly with age, matters more than the quantity.
This is why timing matters. A 32-year-old with low ovarian reserve has better treatment prospects than a 41-year-old with the same hormone levels, even though both would be described as having low fertility. The earlier the issue is identified, the more options remain on the table.