Fertility and trying to conceive

The baseline numbers

For a couple in their twenties with no fertility problems, the chance of conceiving in any given cycle is roughly 20–25 percent. Cumulatively, about 85 percent conceive within 12 months of trying and around 92 percent within 24 months.

This means most of the first year of not conceiving is statistically unremarkable, and also that a year without success is a reasonable point to seek assessment.

Timing

The fertile window is about six days: the five days before ovulation and the day of ovulation itself, because sperm survive up to five days in fertile cervical mucus while the egg survives 12–24 hours.

Wilcox et al. (2000, BMJ) found the window's timing is far more variable than the day-14 model suggests: only about 30 percent of women had their fertile window entirely within days 10–17.

Practical approach: sex every 1–2 days across the fertile window, or simply every 2–3 days throughout the cycle, which removes the need to time anything and performs comparably in the evidence. Daily sex does not meaningfully deplete sperm counts in men with normal parameters.

Ovulation signs: clear, stretchy, slippery cervical mucus (the most useful predictor); a temperature rise of 0.2–0.5 °C after ovulation, which confirms rather than predicts; and LH urine tests detecting the surge 24–36 hours ahead.

Myths worth discarding: specific positions do not matter, lying with legs raised afterwards has no evidence, and orgasm is not required for conception.

Lubricant does matter. Most conventional lubricants impair sperm motility in vitro. Use a fertility-friendly, sperm-safe preparation, or none, if lubricant is needed while trying.

Age

The single largest factor, and the one most often understated.

Egg quality as well as quantity declines, raising miscarriage and chromosomal abnormality rates. Male fertility also declines with age, though more gradually: paternal age over 40–45 is associated with reduced fertility, longer time to conception, higher miscarriage rates and a small increase in some conditions in offspring.

What affects fertility, with evidence

Both partners: smoking (clearly harmful to both egg and sperm quality), significant alcohol intake, obesity and being significantly underweight, poorly controlled chronic disease, some medications, and untreated STIs — chlamydia in particular causes tubal factor infertility, frequently with no symptoms at the time.

Sperm specifically: heat exposure from hot tubs, saunas and laptops on the lap; anabolic steroids and testosterone, which suppress sperm production and can cause prolonged or permanent infertility; varicocele; and some occupational chemical exposures. Sperm production takes roughly 72–90 days, so changes take about three months to show in a semen analysis.

Eggs and ovulation: PCOS, which is the most common cause of anovulation; thyroid dysfunction; hyperprolactinaemia; premature ovarian insufficiency; and endometriosis.

Structural: tubal blockage, fibroids distorting the cavity, uterine anomalies, and adhesions from previous surgery or infection.

Folic acid 400 micrograms daily from before conception reduces neural tube defects; higher doses are advised in specific circumstances. This is one of the few supplements with strong evidence. Most fertility supplements marketed to couples have little or none.

When to seek assessment

• Under 35: after 12 months of regular unprotected sex.
• 35–39: after 6 months.
• 40 or over: straight away.
• Immediately at any age with: irregular or absent periods, known endometriosis or PCOS, previous pelvic surgery or pelvic inflammatory disease, previous cancer treatment, two or more miscarriages, undescended testes or testicular surgery, or a known genetic condition.

Investigation covers both partners from the start. Male factor is the sole or contributing cause in around 40–50 percent of cases, and a semen analysis is quick, cheap and non-invasive — it should not be the last test done.

Options

Ovulation induction, intrauterine insemination, IVF, ICSI, donor gametes, surrogacy and adoption. Access, funding and legal frameworks vary enormously by country and by relationship structure; single people and same-sex couples face additional and frequently unjustified barriers.

Success rates are strongly age-dependent and clinics report them differently — ask for live birth rate per cycle started, for your age group, not pregnancy rate per embryo transfer.

The strain on intimacy

Timed intercourse turns sex into a task with a deadline and a pass/fail result, and it is one of the most commonly reported causes of sexual difficulty in fertility clinics — erectile difficulty and loss of desire specifically around the fertile window are so common as to be expected.

What helps: sex every 2–3 days throughout the cycle rather than concentrated timing; deliberately having sex outside the window with no reproductive purpose; naming the pressure out loud rather than pretending it is not there; and taking planned breaks. Fertility counselling is available at most clinics and is worth using early rather than as a last resort.

Sources

• NICE. Guideline CG156: Fertility problems, assessment and treatment.
• Wilcox AJ, Weinberg CR, Baird DD. "Timing of sexual intercourse in relation to ovulation." NEJM, 1995; and "The timing of the fertile window." BMJ, 2000.
• Practice Committee of the American Society for Reproductive Medicine. "Optimizing natural fertility." Fertility and Sterility, current edition.
• Agarwal A et al. "Male infertility." Lancet, 2021.
• Sharma R et al. "Effects of increased paternal age on sperm quality, reproductive outcome and associated epigenetic risks." Reproductive Biology and Endocrinology, 2015.
• Steiner AZ, Jukic AM. "Impact of female age and nulligravidity on fecundity in an older reproductive age cohort." Fertility and Sterility, 2016.

Last reviewed: August 2026.