Fertility depends on a surprisingly long list of factors, from age and body weight to chemical exposures and the timing of intercourse within a single menstrual cycle. Some of these you can change, others you can’t, but understanding each one gives you a clearer picture of what’s working for or against conception. Here’s what the evidence shows.
Age Is the Single Biggest Factor
For women, fertility begins declining meaningfully in the early to mid-30s. About one third of couples will have difficulty conceiving when the female partner is 35 or older. The reason is straightforward: women are born with a fixed number of eggs, and both the quantity and quality of those eggs drop over time. Doctors can estimate remaining egg supply by counting small follicles visible on an ultrasound or by measuring certain hormone levels early in the menstrual cycle. Abnormal results on these tests suggest reduced fertility potential, though they can’t predict with certainty whether any individual woman will conceive.
Male fertility also declines with age, just more gradually. Sperm DNA damage increases over time. In a study of more than 2,100 semen samples, men 35 and younger had an average DNA fragmentation rate of about 14.7%, compared to 15.9% in men aged 36 to 44 and 16.2% in men 45 and older. Semen volume and sperm motility (how well sperm swim) both decrease noticeably by age 50. These changes don’t make conception impossible, but they do lower the odds and may increase the risk of miscarriage.
Body Weight and Ovulation
Being significantly overweight disrupts the hormonal signals that trigger ovulation. Women with a BMI of 32 or higher at age 18 face roughly 2.7 times the risk of anovulatory infertility, meaning their ovaries stop releasing eggs regularly. Even among women who do ovulate, the chance of spontaneous conception drops by about 5% for every single-point increase in BMI. That means a woman with a BMI of 35 faces a meaningfully lower probability of conceiving in any given cycle than a woman at 25, even if both are ovulating.
Being significantly underweight creates problems too. Low body fat can suppress the hormones needed to maintain a regular cycle, leading to irregular or absent periods. The practical takeaway is that moving toward a moderate weight in either direction can improve ovulatory function, sometimes substantially.
PCOS and Hormonal Imbalances
Polycystic ovary syndrome is the most common cause of ovulation-related infertility, responsible for 90 to 95% of cases among women seeking treatment for anovulation. In PCOS, levels of luteinizing hormone run high while follicle-stimulating hormone stays low, and androgen (male hormone) levels are elevated. This combination prevents eggs from maturing and being released on a normal schedule.
Insulin resistance plays a central role. Roughly 65 to 70% of women with PCOS have insulin resistance regardless of whether they’re overweight. Excess insulin interferes with the liver’s production of a protein that binds sex hormones, which in turn raises the levels of free androgens circulating in the blood. That’s why improving insulin sensitivity through weight loss, dietary changes, or medication can restore ovulation in many women with PCOS.
The Fertile Window
Conception is only possible during a six-day window that ends on the day of ovulation. A landmark study in the New England Journal of Medicine found that the probability of conception ranged from about 10% when intercourse occurred five days before ovulation to 33% on the day of ovulation itself. Outside this window, conception simply doesn’t happen.
This means tracking ovulation matters more than frequency of intercourse. Ovulation predictor kits, basal body temperature charting, and cervical mucus changes can all help identify this window. For couples with regular cycles, having intercourse every one to two days in the days leading up to expected ovulation covers the highest-probability period.
Alcohol and Caffeine
Alcohol has a clear, dose-related negative effect on the ability to conceive. In one study tracking conception rates per menstrual cycle, women who abstained from alcohol and drank less than one cup of coffee per day conceived at a rate of about 27 pregnancies per 100 cycles. Women who consumed any alcohol and more than one cup of coffee per day conceived at a rate of just 10.5 per 100 cycles.
Caffeine on its own didn’t independently reduce the odds of conception in that study, but it appeared to amplify alcohol’s negative effect. For women actively trying to conceive, avoiding alcohol entirely offers the clearest benefit. Moderate caffeine intake (roughly one cup of coffee a day) does not appear to be a problem on its own.
Chemical Exposures
Phthalates, a group of chemicals found in plastics, personal care products, and food packaging, are linked to lower ovarian reserve over time. Research following women over several years found that higher exposure to certain phthalate compounds during pregnancy was associated with lower levels of anti-Müllerian hormone (a marker of remaining egg supply) six and nine years later. The effect was small per individual chemical but consistent across multiple phthalate types.
Bisphenol A (BPA), another widely discussed chemical, did not show a significant association with ovarian reserve markers in the same research. That doesn’t mean it’s harmless in all contexts, but the strongest evidence for everyday chemical exposure affecting fertility currently points to phthalates rather than BPA. Reducing exposure means choosing fragrance-free personal care products, avoiding heating food in plastic containers, and opting for glass or stainless steel when possible.
Heat and Male Fertility
Sperm production requires a temperature slightly below core body temperature, which is why the testes sit outside the body. Even moderate, repeated heat exposure can cause significant damage. In one controlled study, men who sat in a 43°C (about 109°F) water bath for 30 minutes a day over 10 consecutive days experienced serious drops in sperm quality. The good news: the damage was fully reversible within a 16-week recovery period.
Common sources of scrotal heat include laptops placed directly on the lap, prolonged hot tub use, tight-fitting underwear, and extended periods of sitting (long-haul driving, for example). For men trying to conceive, reducing these exposures is a simple, low-cost step. If heat exposure has been a factor, sperm quality typically rebounds within about three to four months, which aligns with the roughly 74-day cycle of new sperm production.
Nutrition Before Conception
Folic acid is the most well-established preconception supplement. The U.S. Preventive Services Task Force recommends that women planning pregnancy take 400 to 800 micrograms daily. Folic acid supports cell division during the earliest stages of embryonic development, specifically the closure of the neural tube, which forms the brain and spinal cord. This process happens in the first few weeks after conception, often before a woman knows she’s pregnant, so starting supplementation before trying to conceive is the point.
For men, zinc and folate both play roles in sperm production and DNA integrity. A balanced diet rich in leafy greens, legumes, nuts, and whole grains covers much of the nutritional ground for both partners. Severe deficiencies in key nutrients can impair fertility, but for people eating a reasonably varied diet, megadosing supplements beyond standard recommendations has not been shown to improve outcomes.
Other Medical Conditions
Beyond PCOS, several other conditions directly affect fertility. Endometriosis, where tissue similar to the uterine lining grows outside the uterus, can block or damage the fallopian tubes and create an inflammatory environment hostile to eggs and sperm. Thyroid disorders, both overactive and underactive, disrupt the hormonal cascade needed for regular ovulation. Sexually transmitted infections, particularly chlamydia and gonorrhea, can cause scarring in the fallopian tubes if left untreated.
In men, varicoceles (enlarged veins in the scrotum) are found in about 40% of men evaluated for infertility. They raise testicular temperature and can reduce sperm count and motility. Untreated diabetes, certain autoimmune conditions, and some medications (including certain antidepressants and testosterone supplementation) can also impair sperm production. Paradoxically, taking external testosterone shuts down the body’s own sperm-producing signals, sometimes reducing sperm counts to zero.