Trying to Conceive: What the Evidence Actually Shows

The biology of conception gets surprisingly little airtime, with the result that many couples spend the first few months either anxious because pregnancy did not happen in cycle one, or quietly missing the actual fertile window because they have been timing things off a "day 14" they read on a forum. Both are easy to fix once you have the underlying numbers and the simple practical rules.

This is what the evidence actually shows about how long it takes, what changes that, and what the high-yield preconception steps are.

Healthbooq (healthbooq.com/apps/healthbooq-kids) covers pregnancy planning, reproductive health, and parenting through pregnancy. For a wider overview, see our complete guide to parenting.

How Conception Actually Works

The egg lives 12 to 24 hours after release. Sperm can survive in the reproductive tract for up to 5 days under good conditions. Add those together and the fertile window is roughly the 6 days ending on the day of ovulation — but the conception probability is far from even across that window. The best three days are the two days before ovulation and the day of ovulation itself; the day after ovulation is essentially a closed window.

In a textbook 28-day cycle, ovulation falls around day 14 (counting day 1 as the first day of the period). In real life, cycles vary. A 32-day cycle ovulates around day 18; a 24-day cycle around day 10. Couples timing intercourse to "day 14" on a 32-day cycle are missing by four days — the most common cause of well-meant but mistimed effort.

The luteal phase (ovulation to next period) is fairly fixed at around 14 days. So the simpler rule is: ovulation happens roughly 14 days before the next period, and that is what to count back from if your cycles are predictable.

The Practical Plan: Sex Every Other Day

The simplest evidence-based approach: have unprotected sex every other day from around day 8 through day 21 of a typical cycle. That covers the fertile window for most cycle lengths, requires no tracking, and avoids the trap of one perfectly timed encounter.

Two myths worth dropping:

• "Saving up" sperm by abstaining for several days does not improve quality. Sperm quality is actually best with ejaculation every 2 to 3 days. Long abstinence increases the proportion of older, less motile sperm.
• Sex more than once a day during the fertile window does not meaningfully increase conception odds and is not necessary.

If timing is a useful add-on:

• Cervical mucus. In the days before ovulation, oestrogen makes mucus clear, slippery, and stretchy — like raw egg white. The day with the most of this fertile-quality mucus is your best signal that ovulation is approaching. Free, accurate with practice, well validated since the Billings work in the 1970s.
• Ovulation predictor kits (OPKs). Detect the LH surge that precedes ovulation by 24 to 36 hours. Cheap urine sticks; digital ones are easier to read. The single most practical real-time method.
• Basal body temperature. Useful for confirming that ovulation happened (a 0.2 to 0.5°C rise after ovulation, sustained), but retrospective — no use for catching this cycle's window.

Apps that calculate ovulation only from cycle dates are crude — they assume average ovulation timing. Apps that incorporate temperature, OPK results, or mucus observations are much more accurate.

How Long It Tends to Take

For a healthy couple in their late 20s having regular unprotected sex:

• Roughly 20 to 25 percent chance per cycle.
• About 60 percent within 6 months.
• About 84 percent within 12 months.
• About 92 percent within 24 months.

Most couples take several months. A negative test in cycle two is not a sign of a problem — it is the expected outcome 75 to 80 percent of the time.

Age, Honestly

Age affects fertility, more in women than men, and the curve steepens after 35. Useful numbers from Leridon's INED demographic data:

• Age 25: about 75 percent conceive within 12 months.
• Age 30: about 70 percent.
• Age 35: about 66 percent, with miscarriage rates also rising.
• Age 38: about 44 percent.
• Age 40 onwards: the chance per cycle drops further and miscarriage rates climb significantly.

The biology is mostly egg quality — the proportion of chromosomally abnormal eggs rises with age, which explains both lower implantation rates and higher early miscarriage rates. Sperm parameters decline more slowly with age, but advanced paternal age does have measurable effects on sperm quality and some pregnancy outcomes.

This is information rather than a verdict. Plenty of women conceive easily after 35, and plenty struggle in their 20s. The relevance is to the threshold for investigation rather than to whether to try.

The Two High-Yield Preconception Steps

Folic acid. Take 400 micrograms daily, starting at least a month before trying and continuing through the first 12 weeks of pregnancy. Neural tube defects (spina bifida, anencephaly) form in the first four weeks of pregnancy — typically before a missed period — so by the time someone tests positive, the window for prevention is largely closed.

A higher dose (5 mg, on prescription) is recommended if there is a personal or family history of neural tube defects, BMI over 30, diabetes, sickle cell disease, coeliac disease, certain anticonvulsants (carbamazepine, valproate), or methotrexate.

Stop smoking. The single biggest reversible lifestyle factor on both male and female fertility. Smoking reduces ovarian reserve, damages sperm DNA, and once pregnant raises miscarriage, preterm birth, low birth weight, and stillbirth rates. The effect is dose-dependent and reversible: cutting down helps, stopping helps a lot more. NHS smoking cessation services are free and effective.

The Other Lifestyle Levers

• Alcohol. NICE advises abstaining from alcohol when trying to conceive — partly because pregnancy may be confirmed weeks after conception. There is no evidence moderate drinking before conception delays conception in healthy women, but there is also no known safe lower limit in early pregnancy. Heavy drinking does affect fertility in both men and women.
• Caffeine. Modest amounts are fine. NHS advice in pregnancy is to keep caffeine under 200 mg/day (roughly two mugs of instant coffee or one strong filter coffee). Pre-conception, similar moderation is reasonable.
• Weight. BMI under 18.5 or over 30 can affect ovulation. In women with BMI above 30 who are not ovulating, even a 5 to 10 percent weight loss can restore ovulatory cycles. Severe undernutrition can stop periods entirely.
• Exercise. Moderate exercise helps. Very heavy training (typical of elite endurance athletes) can suppress ovulation; if periods have stopped or become irregular with training, that is worth medical review.
• Drugs and medications. Stop any recreational drugs. Review current prescriptions with the GP — some medications are safe in pregnancy, some need swapping (for example, certain antidepressants, some anticonvulsants), and some are absolutely contraindicated (methotrexate, isotretinoin, some hypertension medications).
• Vaccinations. Check rubella immunity (asked at booking but useful to confirm before conception — MMR is a live vaccine and should not be given in pregnancy, so any required dose should be given before conception with a one-month gap). Whooping cough vaccination happens in pregnancy itself.
• Vitamin D. All adults in the UK are advised 10 micrograms (400 IU) daily, especially through autumn and winter. Continues through pregnancy and breastfeeding.

Prenatal vitamins beyond folic acid (and vitamin D in winter) add modest extra value for most healthy women eating a varied diet — they are not harmful but folic acid is the part with strong evidence.

Male Factor — Often Overlooked

Around 40 to 50 percent of subfertility involves a male factor, either alone or in combination. The simplest test is a semen analysis: volume, concentration, motility, and morphology. It is non-invasive, cheap, and frequently reveals a problem that explains months of trying without success.

For male preconception:

• Avoid hot tubs and saunas regularly — heat reduces sperm production.
• Stop smoking and recreational drugs.
• Limit alcohol.
• Manage chronic conditions (diabetes, obesity, hypertension).
• Anabolic steroids and testosterone supplements suppress sperm production — sometimes for many months after stopping.

When to Seek Investigation

NICE recommends:

• Couple under 35 with regular unprotected sex: investigate after 12 months.
• Woman 35 or over: investigate after 6 months.
• Earlier referral if there is a clear reason — known endometriosis, prior pelvic infection, irregular or absent periods, prior cancer treatment, undescended testes, or other concerns.

Initial investigations are straightforward:

• Day 2–5 hormone profile (FSH, LH, oestradiol) — ovarian reserve and pituitary function.
• Day 21 progesterone (or "luteal phase" progesterone, 7 days before expected period) — confirms ovulation occurred.
• AMH (anti-Müllerian hormone) — ovarian reserve marker.
• Thyroid function and prolactin if periods are irregular.
• Semen analysis for the male partner.
• Pelvic ultrasound, and sometimes a hysterosalpingogram or HyCoSy to check tubal patency, depending on findings.

Many fertility issues identified at this stage have specific, often non-IVF treatments — ovulation induction with letrozole or clomifene, surgical correction of fibroids or polyps, treatment of endometriosis, or simple optimisation of sperm quality. Investigation is not the same as IVF.

A Simple Plan If You Are Just Starting

• Start folic acid 400 mcg daily today.
• Stop smoking, cut back on alcohol, and review any medications with your GP.
• Have unprotected sex every other day from around day 8 to day 21 of the cycle, or every 2 to 3 days continuously if the cycle is irregular.
• Track cycles for a few months to identify your ovulation pattern; consider OPKs from cycle 3 if helpful.
• Test from the day of a missed period; do not test earlier and read the same line three times.
• Be patient through 6 months. Investigate at 12 months (or 6 months over 35) if it has not happened.

Most couples conceive within the first year. The minority who do not have, in the great majority of cases, an identifiable issue with a real path forward. Either way, you are not alone, and the steps from here are usually clearer than they feel from cycle four.

About 84 percent of couples conceive within 12 months and 92 percent within 24 months when having regular unprotected sex. The fertile window is around 6 days, ending on the day of ovulation, with the best chance from sex on the 2 to 3 days before ovulation. Monthly conception probability is 20 to 25 percent in healthy couples in their late 20s. The most important preconception step is folic acid 400 mcg daily, starting at least a month before trying and continuing through the first 12 weeks. Investigation is recommended after 12 months of trying — 6 months if the woman is over 35.