The decision to try for a baby is one thing; understanding what trying actually means in practice is another. Most couples have very little accurate information about the biology of conception, the timing of ovulation, or what is and isn't normal in terms of how long it takes. The result is either unnecessary anxiety when pregnancy doesn't happen in the first month, or missing genuinely important steps like folic acid supplementation that should start before pregnancy begins.
This article covers what the evidence actually shows about conception timing, the factors that affect fertility, and when to start looking into further investigation.
Healthbooq (healthbooq.com/apps/healthbooq-kids) covers pregnancy planning and reproductive health.
The Basics of the Fertile Window
Conception requires the fertilisation of an egg by a sperm. An egg is released at ovulation and is viable for around 12-24 hours. Sperm, however, can survive in the reproductive tract for up to 5 days under good conditions. This means the fertile window extends from around 5 days before ovulation to the day of ovulation itself – a window of approximately 6 days, though the probability of conception is highest in the 2-3 days leading up to and including ovulation.
In a textbook 28-day cycle, ovulation occurs around day 14. In reality, cycle length varies considerably between women and, in the same woman, between cycles. A woman with a 32-day cycle typically ovulates around day 18; a woman with a 24-day cycle ovulates around day 10. Using average figures without knowing individual cycle length is one of the most common sources of mistimed intercourse.
Signs of Ovulation and Tracking Methods
Several approaches help identify the fertile window:
Basal body temperature (BBT) tracking involves taking temperature first thing in the morning (before getting up), using a sensitive basal body thermometer. BBT rises by 0.2-0.5 degrees Celsius after ovulation (due to progesterone), confirming that ovulation has occurred – but the rise is retrospective, so BBT is more useful for understanding the cycle pattern over several months than for pinpointing the current cycle's fertile window.
Cervical mucus observation is a reliable real-time indicator. In the days approaching ovulation, oestrogen causes cervical mucus to become increasingly clear, slippery, and stretchy – often described as similar to raw egg white. This "egg white cervical mucus" (EWCM) is the clearest natural signal that ovulation is approaching. Monitoring this change takes practice but is free and highly informative. Research by Billings and colleagues established cervical mucus observation as a reliable natural fertility sign in the 1970s; subsequent studies have confirmed it.
Ovulation predictor kits (OPKs) detect the LH surge that precedes ovulation by 24-36 hours. The standard urine test is reliable and inexpensive; digital kits provide a clearer reading. OPKs give advance warning of the fertile window and are probably the most widely used approach in couples actively trying to conceive.
There is no evidence that having sex more than once every 24-48 hours during the fertile window significantly improves conception rates, and abstaining to "save" sperm for several days is counterproductive: sperm quality is optimised with ejaculation every 2-3 days.
How Long Does It Take?
For a healthy couple in their late 20s to early 30s having regular (2-3 times per week) unprotected sex, the probability of conception in any given cycle is around 20-25%. Around 84% of couples conceive within 12 months, and around 92% within 24 months. That a pregnancy doesn't happen in the first few months is not a sign that something is wrong: statistically, most couples take several months.
The monthly probability of conception (fecundability) declines with age, particularly for women. Research by Henri Leridon at the Institut National d'Études Démographiques in France, using historical European birth data, showed that at age 25, around 75% of women conceive within 12 months; by age 35, this falls to around 66%; by age 38, around 44%. The decline accelerates from 35 onwards and is primarily driven by the increasing rate of chromosomally abnormal eggs.
Male factor contributes to around 40-50% of subfertility cases – a figure that surprises many people, given the assumption that subfertility is primarily a female issue. Semen analysis (volume, concentration, motility, morphology) is the primary investigation.
Preconception Steps
The most important preconception health step is taking folic acid. Neural tube defects (spina bifida, anencephaly) develop in the first four weeks of embryonic development – before most women know they are pregnant. Taking 400 micrograms of folic acid daily for at least a month before conceiving and for the first 12 weeks of pregnancy significantly reduces the risk. Women with a personal or family history of neural tube defects, or who take certain medications (including anticonvulsants and methotrexate), need a higher dose (5mg/day, on prescription).
Stopping smoking is the most impactful lifestyle change for both male and female fertility. Smoking reduces ovarian reserve, damages sperm DNA, and, once pregnant, significantly increases the risks of miscarriage, preterm birth, and low birth weight. The effect is dose-dependent and reversible: stopping completely is what makes the difference.
Alcohol in pregnancy is the subject of much advice, but preconception advice is less clear. NICE advises abstaining from alcohol when trying to conceive, primarily because the embryo's development begins before pregnancy is confirmed. There is no evidence that moderate alcohol consumption affects time to conception in healthy women, but the caution is reasonable given the absence of a known safe lower limit in pregnancy.
Body weight: both underweight (BMI under 18.5) and overweight (BMI over 30) can affect ovulation and reduce fertility. The effect of overweight is primarily mediated through hormonal disruption – excess adipose tissue increases oestrogen and can disrupt the HPO axis. Modest weight loss in women with BMI over 30 who are not ovulating can restore ovulatory cycles.
Starting prenatal vitamins before conception is reasonable but the additional value beyond folic acid is modest for most healthy women eating a varied diet.
When to Seek Further Assessment
NICE guidelines recommend that a couple should seek investigation after 12 months of regular unprotected sex if the woman is under 35. For women 35 or over, investigation should begin after 6 months. This earlier threshold reflects the more rapid age-related fertility decline and the narrower window for investigation and treatment.
Investigation at this point is not necessarily the prelude to IVF. A significant proportion of subfertility has relatively straightforward causes: ovulatory problems (diagnosed by blood tests), structural issues detectable on ultrasound or hysteroscopy, or sperm quality problems. Many are treatable without assisted conception.
The first tests are: day 2-5 hormone profile (FSH, LH, oestradiol) to assess ovarian reserve and ovulation; day 21 progesterone (to confirm ovulation has occurred in a 28-day cycle); AMH (anti-Müllerian hormone) as an ovarian reserve marker; semen analysis; and, if clinically indicated, pelvic ultrasound.
Anyone concerned about delayed conception who wants a reliable starting point can consult their GP, who will arrange these initial investigations.
Key Takeaways
Most couples in the UK who are trying to conceive do so within a year: around 84% of couples conceive within 12 months if they have regular unprotected sex. Natural conception depends on correctly timed intercourse around ovulation, which occurs approximately 14 days before the next period in a regular 28-day cycle but varies significantly between women. Key preconception steps include taking folic acid before pregnancy begins, stopping smoking, and managing any existing health conditions. Age is the most important biological factor affecting fertility, particularly for women, with a decline accelerating from around age 35. Subfertility investigation should begin after 12 months of trying (or 6 months for women over 35).
