Fertility depends on a combination of timing, body weight, diet, sleep, physical activity, and environmental exposures. Some of these factors have a surprisingly large effect. For healthy couples in their 20s and early 30s, the chance of conceiving in any given cycle is about 1 in 4. By age 40, that drops to roughly 1 in 10. But age aside, there’s a lot within your control that can shift those odds meaningfully.
Timing Intercourse to the Fertile Window
Conception can only happen 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. The two to three days leading up to ovulation are the sweet spot, since sperm can survive in the reproductive tract for up to five days but an egg is only viable for about 12 to 24 hours after release.
Tracking ovulation through basal body temperature, cervical mucus changes, or ovulation predictor kits helps you identify this window more reliably than calendar counting alone. Having intercourse every one to two days during the fertile window gives sperm the best chance of being in the right place at the right time.
How Body Weight Affects Conception
Both ends of the weight spectrum reduce fertility. Women who are overweight or obese before pregnancy take measurably longer to conceive and have a higher risk of what researchers call “subfecundity,” or reduced ability to get pregnant within a year. Being underweight causes its own problems: it can trigger irregular or absent periods and shorten the luteal phase (the critical second half of the cycle when a fertilized egg needs to implant), both of which make conception harder.
A BMI in the normal range, roughly 18.5 to 24.9, is associated with the shortest time to pregnancy. Interestingly, a partner’s weight matters too. Research from a large cohort study found that the negative effect of a woman’s higher BMI on time to pregnancy was partly mediated by her partner’s BMI, suggesting that weight management as a couple can make a difference.
The Mediterranean Diet and Live Birth Rates
What you eat has a direct relationship to both egg and sperm quality. The Mediterranean diet, built around fruits, vegetables, whole grains, olive oil, and fish, is the most studied dietary pattern in fertility research, and the results are striking. In studies of couples undergoing fertility treatment, women with the highest adherence to this eating pattern had a 49% live birth rate compared to 27% among those with the lowest adherence.
The mechanisms are fairly straightforward. Olive oil and fish provide fats and omega-3 fatty acids that improve sperm quality. Fruits and vegetables deliver antioxidants that protect eggs from oxidative damage. Whole grains contribute B vitamins and fiber that support hormonal balance. You don’t need to follow the diet perfectly. Even moving from low to moderate adherence is associated with meaningful improvements in pregnancy rates.
Key Supplements for Preconception
Folic acid is the single most important preconception supplement. The U.S. Preventive Services Task Force recommends 400 to 800 micrograms daily, starting at least one month before conception and continuing through the first two to three months of pregnancy. This prevents neural tube defects, and the recommendation applies to anyone who could become pregnant, not just those actively trying.
Coenzyme Q10 (CoQ10) has gained attention for its role in egg quality, particularly for women over 35 or those with diminished ovarian reserve. It supports the energy-producing structures inside eggs, which become less efficient with age. Clinical studies have used doses of 200 mg three times daily for 60 days in women with low ovarian reserve, with improvements in ovarian response and clinical pregnancy rates. For women with normal ovarian reserve, 200 mg daily for 30 to 35 days before trying to conceive is the dose most commonly studied.
Sleep and Reproductive Hormones
Your reproductive hormones follow a 24-hour rhythm, and disrupting that rhythm disrupts fertility. The monthly surge of luteinizing hormone and follicle-stimulating hormone that triggers ovulation occurs primarily during the later hours of the dark phase of your sleep cycle. Melatonin, the hormone your body produces in darkness, does more than regulate sleep. It directly influences progesterone production and correlates with markers of ovarian reserve.
Shift workers, who experience chronic circadian disruption, have well-documented increases in menstrual irregularity and fertility problems. Even without shift work, consistently poor or short sleep can throw off the hormonal cascade that makes ovulation possible. Aiming for seven to nine hours on a regular schedule, in a dark room, supports both melatonin production and the downstream reproductive hormones that depend on it.
Exercise: How Much Is Too Much
Moderate exercise improves fertility, but vigorous exercise can start working against you at surprisingly low thresholds. In a large study adjusting for body weight, women who did just two hours per week of vigorous exercise were 16% less likely to conceive than sedentary women. At three to four hours weekly, that figure rose to 27%, and at five or more hours it reached 32%.
The underlying issue is energy balance, not exercise itself. When the body senses a caloric deficit, it suppresses reproductive function. In one controlled study, 85% of women placed in moderate to severe caloric deficit while exercising vigorously experienced luteal phase defects, meaning their cycles appeared normal on the surface but the hormonal environment couldn’t support implantation. Walking, yoga, swimming, and moderate-intensity activity don’t carry these risks and are actively beneficial. If you’re a heavy exerciser trying to conceive, reducing intensity or increasing calorie intake can restore normal cycles relatively quickly.
Male Fertility Factors
Fertility is a two-person equation, and male factors contribute to roughly half of all cases of difficulty conceiving. Three lifestyle factors have the clearest evidence behind them.
Heat is the most underappreciated one. Even a 1 to 1.5 degree Celsius rise in scrotal temperature impairs sperm production and causes abnormal sperm shape. Tight underwear, frequent hot tub use, and even laptop use on the lap all raise scrotal temperature enough to measurably reduce sperm count and motility. Sperm counts are consistently lower in summer months and higher in winter, reflecting this sensitivity.
Smoking has a dose-dependent effect. A meta-analysis of nearly 6,000 men found that smoking reduces both sperm count and motility, with the damage proportional to the number of cigarettes smoked per day. The good news: quitting has been shown to improve sperm concentration, semen volume, and total sperm count.
Daily alcohol consumption affects semen volume, sperm shape, and hormone balance, and lengthens time to pregnancy. Chronic heavy drinking combined with smoking is particularly damaging, lowering testosterone levels and compounding semen quality problems. Moderate drinking, on the other hand, does not appear to have significant adverse effects.
Reducing Exposure to Endocrine Disruptors
Chemicals that mimic estrogen or interfere with hormone signaling are present in plastic containers, food can linings, personal care products, cosmetics, and detergents. The two most studied classes are phthalates and bisphenols (like BPA). These chemicals enter your body through food, skin contact, and even inhalation.
In the reproductive system, they cause specific, measurable harm. BPA mimics estrogen, binding to estrogen receptors and disrupting the hormonal signals that drive follicle growth. It also interferes with the production of estrogen itself in the cells surrounding developing eggs. Phthalates trigger oxidative stress inside follicles, increasing cell death signals and reducing estrogen production. Both chemicals can alter the timing of the luteinizing hormone surge, directly interfering with ovulation.
Practical steps to reduce exposure include switching from plastic to glass food storage containers, avoiding heating food in plastic, choosing fragrance-free personal care products (synthetic fragrances often contain phthalates), and opting for fresh or frozen foods over canned. You can’t eliminate exposure entirely, but reducing it lowers the total burden on your hormonal system.
Age and What You Can Do About It
Egg quantity and quality decline with age, and this is the single biggest factor in fertility that you cannot reverse. The drop accelerates after 35 and becomes steep after 40. But the lifestyle factors above matter more, not less, as you get older, because they influence the quality of the eggs you do have. A 37-year-old with excellent sleep, a nutrient-dense diet, healthy weight, and low toxin exposure has meaningfully better odds than a 37-year-old without those advantages. The biology of aging can’t be stopped, but the environment your eggs develop in over the three months before ovulation is something you can actively shape.