Even with perfectly timed intercourse, a healthy woman under 30 has only about a 20–25% chance of conceiving in any given cycle. That means several months of trying with no result is completely normal. Infertility isn’t formally defined until you’ve been trying for 12 months without success (or 6 months if you’re 35 or older). But whether you’re just starting to worry or you’ve been at this for a while, understanding the most common reasons conception doesn’t happen can help you figure out what to do next.
Your Odds Change Significantly With Age
Age is the single biggest factor in monthly conception rates, and the decline is steeper than most people expect. At 25, a healthy woman has roughly a 25% chance of getting pregnant each cycle. By 30, that drops to about 20%. At 35, it’s around 15%. And by 40, the chance per cycle falls to approximately 5%.
This decline reflects egg quality more than egg quantity, though both matter. As eggs age, they’re more likely to have chromosomal abnormalities that prevent a viable pregnancy or lead to early miscarriage. This is why the clinical timeline for seeking help is shorter for women over 35: six months of trying rather than a full year.
Ovulation Problems Are the Most Common Cause
If you’re not ovulating regularly, conception can’t happen. Irregular or absent periods are the most obvious sign, but some women ovulate inconsistently even with cycles that seem normal. Polycystic ovary syndrome (PCOS) is the most well-known cause, affecting roughly one in 15 women of reproductive age. PCOS disrupts the hormonal signals that trigger ovulation, often causing irregular cycles, excess body hair, or acne alongside fertility issues.
Thyroid problems also interfere with ovulation. Even mild thyroid dysfunction can make conception harder. The American Thyroid Association recommends keeping TSH levels below 2.5 during pregnancy for women on thyroid medication, and women with an autoimmune thyroid condition (Hashimoto’s) may need their levels even lower before conception to maintain healthy thyroid function in early pregnancy. If you haven’t had your thyroid checked, it’s one of the first blood tests worth requesting.
It’s Not Always a Female Factor
Male factor infertility contributes to roughly half of all cases where couples can’t conceive, either as the sole cause or in combination with a female factor. A semen analysis is one of the simplest and least invasive fertility tests available, yet it’s often delayed because couples assume the issue lies elsewhere.
The World Health Organization considers sperm concentration normal at 16 million or more per milliliter, with at least 42% of sperm showing movement and at least 4% having normal shape. Falling below these thresholds doesn’t mean pregnancy is impossible, but it reduces the odds each cycle. Common causes of low sperm quality include heat exposure (laptops, hot tubs, tight clothing), varicoceles (enlarged veins in the scrotum), smoking, and certain medications. Some causes are reversible within a few months of lifestyle changes, since the body produces a new batch of sperm roughly every 74 days.
Blocked or Damaged Fallopian Tubes
The fallopian tubes are where egg meets sperm, so any blockage can prevent conception entirely. In one study of 373 women being evaluated for infertility, about 25% had at least one blocked tube. Unilateral blockage (one tube) was roughly twice as common as bilateral (both tubes). With one open tube, pregnancy is still possible but takes longer on average.
The most common cause of tubal blockage is pelvic inflammatory disease, usually the result of untreated chlamydia or gonorrhea, sometimes years earlier. Endometriosis, ruptured appendicitis, and previous pelvic surgery can also cause scarring that blocks or damages the tubes. A test called a hysterosalpingogram (an X-ray with dye injected through the cervix) can identify blockages and is a standard part of a fertility workup.
Endometriosis You Might Not Know About
Endometriosis is often associated with painful periods, but a significant number of women have no pain symptoms at all. In a review of 11 studies covering over 1,700 women with unexplained infertility, 44% were diagnosed with endometriosis after surgical investigation. Most of those cases were minimal or mild (about 74%), which is precisely why they’d gone undetected.
Endometriosis can impair fertility through inflammation that damages eggs or sperm, scarring that distorts the anatomy of the pelvis, or changes to the uterine lining that make implantation harder. If you’ve been given an “unexplained infertility” diagnosis, it’s worth discussing whether endometriosis has been adequately ruled out. Non-surgical screening methods, including specialized ultrasound and newer biomarker tests, are becoming more available.
Timing Matters More Than You Think
The fertile window is surprisingly narrow. Pregnancy is most likely when intercourse happens in the three days before ovulation. The peak day, two days before ovulation, gives roughly a 26% conception rate per cycle. By just one day after ovulation, that rate plummets to about 1%. An egg survives only 12 to 24 hours after release, while sperm can live up to five days in the reproductive tract, which is why sex before ovulation is more effective than sex after.
Many people estimate their ovulation day incorrectly, especially if their cycles vary in length. Ovulation predictor kits that detect the hormonal surge before egg release are more reliable than calendar counting alone. Tracking basal body temperature can confirm that ovulation occurred, but the temperature rise happens after the fact, so it’s more useful for understanding your pattern over several cycles than for timing a specific month.
Weight and Body Composition
Both extremes of body weight affect ovulation. A BMI below 18.5 often causes irregular cycles and can stop ovulation entirely. The body essentially reads low weight as a signal that conditions aren’t safe for pregnancy and dials down reproductive hormones. On the other end, a BMI over 30 also disrupts ovulation through excess production of certain hormones that interfere with the normal cycle.
The good news is that ovulatory function often returns with relatively modest weight changes. For women who are underweight, gaining even a small amount can restart regular cycles. For women in the obese range, losing 5–10% of body weight frequently improves ovulation rates, sometimes enough to conceive without further treatment.
Smoking and Fertility Loss
Smoking accelerates reproductive aging in a way that few other lifestyle factors match. Women who smoke reach menopause one to four years earlier than nonsmokers. A key marker of ovarian reserve (the hormone AMH) is 44% lower in current smokers compared to women who have never smoked, and it declines 21% faster per year. The practical result: smokers experience conception delays over 12 months at a rate 54% higher than nonsmokers.
The damage extends to assisted reproduction as well. Smokers require nearly twice as many IVF cycles to conceive compared to nonsmokers. Secondhand smoke exposure also matters: one study found that passive smokers had reproductive hormone levels 39% higher than unexposed women, a sign of diminished ovarian reserve. Quitting doesn’t reverse all the damage, but it stops the accelerated decline.
When to Start Investigating
The CDC defines infertility as 12 months of unprotected sex without conception. But that timeline shortens to 6 months if you’re 35 or older, and you should seek help sooner at any age if you have irregular periods, a known condition like PCOS or endometriosis, a history of pelvic infections, or prior cancer treatment.
A basic fertility workup typically involves blood tests to check ovulation-related hormones and thyroid function, a semen analysis for your partner, and imaging to evaluate your uterus and fallopian tubes. These three steps identify a cause in the majority of cases. For the roughly 10–15% of couples where no cause is found, further investigation, including screening for silent endometriosis, can often provide answers that shift the treatment approach.