How the fertile window works

Ovulation typically occurs 14 days before the next period (not 14 days after the last one). The fertile window spans roughly the 5 days before ovulation plus ovulation day itself, since sperm can survive up to 5 days in the reproductive tract. An egg survives only 12-24 hours after release, making the day before and day of ovulation the most fertile.

Cycle length and estimated ovulation

Cycle length	Estimated ovulation	Fertile window
24 days	Day 10	Days 5-11
28 days	Day 14	Days 9-15
30 days	Day 16	Days 11-17
35 days	Day 21	Days 16-22

Fertility tracking methods compared

Calendar calculation is the least accurate method (Pearl index varies widely with cycle irregularity). More reliable approaches:

LH predictor kits (OPKs): detect the LH surge 24-36 hours before ovulation. Available over the counter at Boots, Superdrug, and supermarkets without prescription. These are the most accessible method.
Basal body temperature (BBT): temperature rises 0.2-0.5 degrees C after ovulation. Confirms ovulation has occurred but does not predict it in advance.
Cervical mucus monitoring: part of the Fertility Awareness Method (FAM) and Billings method, recognised by the NHS for natural family planning.

Fertility apps: regulated as medical devices in the UK

Since 2021, the MHRA (Medicines and Healthcare products Regulatory Agency) regulates certain fertility apps as Class IIa medical devices if they make clinical claims. Natural Cycles, for example, received MHRA clearance as a contraceptive app. However, the regulatory classification only covers apps making specific contraceptive claims, many cycle-tracking apps remain unregulated consumer products. A 2021 BMJ analysis found that most popular cycle apps had significant accuracy limitations, particularly for women with irregular cycles.

PCOS: the most common cause of irregular cycles in the UK

Polycystic Ovary Syndrome (PCOS) affects approximately 1 in 10 women of reproductive age in the UK, making it the most common cause of irregular or absent ovulation. In PCOS, the LH surge may not follow the regular pattern, making calendar methods particularly unreliable. The NHS recommends LH predictor kits alongside BBT monitoring for women with PCOS trying to conceive. Diagnosis requires a blood test and often an ultrasound.

IVF on the NHS: the postcode lottery

NICE guideline NG156 (2013, updated 2023) recommends up to 3 full IVF cycles on the NHS for women under 40 with a diagnosis of infertility after 2 years of trying. However, individual Integrated Care Boards (ICBs, formerly CCGs) set their own criteria, and provision varies drastically across England:

Some ICBs fund 3 cycles as NICE recommends.
Others fund only 1 cycle, or impose stricter age or BMI criteria.
Scotland follows NHS Scotland guidance (up to 3 funded cycles for eligible couples).
Wales: up to 3 cycles for eligible couples through the All Wales IVF Policy.
Northern Ireland: historically limited funding; NICE guidance is advisory only there.

Disclaimer: This tool provides general information only and is not a substitute for medical advice. Cycle calculations are estimates and cannot predict ovulation with certainty. Consult a GP or fertility specialist for personal guidance.