If you’ve been having regular, unprotected sex and haven’t conceived, you’re not alone. About one in six couples experience difficulty getting pregnant. The medical threshold is 12 months of trying without success if you’re under 35, six months if you’re between 35 and 40, and immediate evaluation if you’re over 40 or have known risk factors like a history of pelvic inflammatory disease or ectopic pregnancy. But whether you’ve hit those timelines or not, understanding the most common reasons conception doesn’t happen can help you figure out your next step.
You May Be Missing the Fertile Window
Conception can only happen during a surprisingly narrow stretch of your cycle. A landmark study in the New England Journal of Medicine found that pregnancies occurred only when intercourse took place during a six-day window ending on the day of ovulation. The probability of conceiving from a single act of intercourse ranged from about 10% five days before ovulation to 33% on ovulation day itself. Outside that window, the chance is essentially zero.
Many people overestimate how long this window lasts or misjudge when it falls. Ovulation doesn’t always happen on day 14, especially if your cycles are irregular. Ovulation predictor kits, which detect a hormone surge in your urine, can help you identify your most fertile days. Having sex every one to two days during that window is sufficient. There’s no evidence that daily intercourse lowers your chances compared to every other day.
Ovulation Problems Are the Most Common Female Factor
If your body isn’t releasing an egg regularly, there’s nothing for sperm to fertilize. The most frequent cause of ovulation failure is polycystic ovary syndrome (PCOS), which affects a significant number of women of reproductive age. In PCOS, hormone levels are disrupted: levels of luteinizing hormone run too high, driving excess production of male hormones (androgens), while follicle-stimulating hormone stays relatively low. That combination stalls the development of egg-containing follicles in the ovaries, so eggs either aren’t released or are released unpredictably.
Signs of PCOS include irregular or absent periods, acne, excess facial or body hair, and thinning hair on the scalp. But not everyone with PCOS has all of these symptoms. If your periods are unpredictable or come fewer than nine times a year, ovulation problems are worth investigating even if you don’t have other obvious signs.
Other causes of ovulation disruption include thyroid disorders, excessive exercise, very low body weight, and elevated levels of the hormone prolactin. Many of these are treatable once identified.
Blocked or Damaged Fallopian Tubes
Your fallopian tubes do more than connect the ovaries to the uterus. They’re where sperm meets egg, where fertilization happens, and where the early embryo develops for its first few days before traveling to the uterus to implant. If one or both tubes are blocked or scarred, this process breaks down.
The two most common causes of tubal damage are pelvic inflammatory disease (usually from untreated chlamydia or gonorrhea infections) and endometriosis. With endometriosis, tissue similar to the uterine lining grows outside the uterus, triggering chronic inflammation. That inflammation produces scar tissue and adhesions that can distort the tubes, block them, or impair their ability to transport an egg. Even when tubes aren’t fully blocked, the inflammatory environment created by endometriosis in the pelvis can interfere with fertilization and implantation. Previous abdominal or pelvic surgeries can also cause adhesions that affect tubal function.
Endometriosis Beyond the Tubes
Endometriosis doesn’t have to block your tubes to reduce fertility. The condition creates a hostile environment for conception in several ways. Endometrial-like tissue outside the uterus triggers a sustained inflammatory response, flooding the pelvic cavity with inflammatory signals. These signals can damage eggs, impair sperm function, and interfere with an embryo’s ability to implant in the uterine wall.
Endometriosis is tricky because its severity doesn’t always match its symptoms. Some women with extensive disease have mild pain, while others with small amounts of displaced tissue have debilitating symptoms. An estimated 30% to 50% of women with endometriosis experience fertility difficulties. If you have painful periods, pain during sex, or chronic pelvic pain alongside trouble conceiving, endometriosis is a real possibility.
Sperm Problems Account for Nearly Half of Cases
Fertility is not solely a female issue. Male factors contribute to roughly 40% to 50% of all infertility cases, either alone or in combination with a female factor. A semen analysis is one of the first tests any couple should pursue, yet it’s often delayed because of assumptions that the problem must be on the female side.
The key measurements in a semen analysis are sperm count, motility (the percentage of sperm that swim forward effectively), and morphology (the percentage with normal shape). The World Health Organization’s reference thresholds set the bar at 15 million sperm per milliliter, at least 32% with forward movement, and at least 4% with normal shape. Falling below any of these increases the difficulty of natural conception. Common causes of poor sperm quality include varicoceles (enlarged veins in the scrotum), hormonal imbalances, excessive heat exposure, smoking, heavy alcohol use, and certain medications. In some cases, no clear cause is found.
Body Weight and Fertility
Your weight has a measurable impact on your ability to conceive. A large meta-analysis found that women with a BMI of 25 or higher were 24% less likely to achieve a clinical pregnancy compared to women at a healthy weight. For women with a BMI of 30 or above, the likelihood dropped by 39%. Higher BMI was also associated with a longer time to pregnancy, with a 32% increased chance of taking more than 12 months to conceive. These findings held even after researchers excluded women with hormonal or gynecological conditions that independently affect both weight and fertility.
Excess body fat disrupts the hormonal signals that control ovulation. Fat tissue produces estrogen, and too much of it can throw off the balance between estrogen, progesterone, and the pituitary hormones that trigger egg release. Being significantly underweight causes problems too, though through a different mechanism: the body essentially shuts down reproductive function when it senses insufficient energy reserves. For men, obesity is linked to lower testosterone levels and reduced sperm quality.
Age Is a Major Factor
Female fertility declines gradually starting in the early 30s and more sharply after 35. This isn’t just about egg quantity, though that matters. Egg quality also drops, meaning a higher percentage of eggs carry chromosomal abnormalities that prevent a viable pregnancy. By age 40, the chance of conceiving naturally in any given cycle is significantly lower than at 30, and the risk of miscarriage rises substantially. This is the reason medical guidelines shorten the recommended “trying” period from 12 months to six months at age 35, and recommend immediate evaluation at 40.
Male fertility also declines with age, though more gradually. Men over 40 tend to have lower sperm quality and longer times to conception, and there’s an increased risk of genetic abnormalities in offspring.
Unexplained Infertility
After completing a full workup, somewhere between 15% and 30% of couples receive a diagnosis of unexplained infertility. This means standard testing, including semen analysis, ovulation testing, ovarian reserve assessment, and imaging of the uterus and fallopian tubes, has come back normal. It doesn’t mean nothing is wrong. It means current tests aren’t sensitive enough to identify the specific problem. Subtle issues with egg quality, sperm function at the molecular level, fertilization itself, or embryo implantation can all escape standard detection.
A diagnosis of unexplained infertility can be frustrating, but it doesn’t mean treatment won’t work. Many couples with this diagnosis go on to conceive with fertility treatments or, in some cases, on their own with more time.
What Testing Looks Like
If you’ve reached the point of seeking medical evaluation, the initial workup is fairly standardized. For the female partner, it typically includes blood tests for follicle-stimulating hormone (FSH) and anti-Müllerian hormone (AMH), both of which give an indication of how many eggs remain in the ovaries. A progesterone blood test midway through your cycle can confirm whether you ovulated. An HSG, or hysterosalpingogram, is an X-ray where contrast dye is pushed through the uterus and fallopian tubes to check for blockages. A transvaginal ultrasound can reveal ovarian cysts, fibroids, or structural issues with the uterus.
For the male partner, the starting point is a semen analysis. It’s noninvasive, inexpensive, and provides a lot of information quickly. If results are abnormal, further testing can look for hormonal causes or structural issues. Because male factor infertility is so common and so easy to screen for, it makes sense to test both partners at the same time rather than completing the full female workup first.