Conception is a narrow biological window. Understanding the physiology of your cycle — rather than just counting days on an app — gives you real information about what's actually happening each month.
Shedding of the endometrial lining secondary to progesterone withdrawal. Day 1 of bleeding is the formal start of your cycle. Normal blood loss: 30–80 mL. >80 mL suggests menorrhagia and warrants investigation.
FSH (follicle-stimulating hormone) from the anterior pituitary drives maturation of a dominant follicle in the ovary. Rising oestradiol from the growing follicle stimulates endometrial proliferation and produces fertile-quality cervical mucus (clear, stretchy — "egg white" consistency).
The LH surge — triggered by peak oestradiol — causes follicular rupture and oocyte release within 36–44 hours. The oocyte is viable for only 12–24 hours post-release. This is your actual fertile day.
The ruptured follicle transforms into the corpus luteum, secreting progesterone to maintain the endometrial lining. If implantation occurs, hCG (produced by the trophoblast) rescues the corpus luteum. The luteal phase is relatively fixed at 12–14 days — cycle length variability occurs in the follicular phase.
Sperm survive in the female reproductive tract for up to 5 days under favourable cervical mucus conditions. Combined with the oocyte's 12–24 hour viability, the true fertile window is approximately 6 days — the 5 days before ovulation plus ovulation day itself.
The highest probability of conception is 1–2 days before ovulation. Having intercourse every 1–2 days during this window is as effective as precisely timed intercourse, with less psychological burden.
Urine LH tests detect the pre-ovulatory LH surge 24–36 hours before ovulation — giving you actionable lead time. Begin testing from Day 10 (earlier if cycles are short). Most reliable method for home use.
As oestrogen rises, cervical mucus transitions from scant/thick to abundant, clear, and stretchy ("spinnbarkeit" — can be stretched between fingers without breaking). This fertile mucus facilitates sperm transport. Learning to track mucus changes is free and surprisingly accurate.
Progesterone raises BBT by 0.2–0.5°C after ovulation. BBT tracking confirms ovulation occurred but is retrospective — it cannot predict the fertile window. Useful as an adjunct, not as a sole method for timing.
💡 App caveat: Cycle-tracking apps predict your fertile window based on your cycle history, not real-time data. They are most reliable in women with very regular cycles. For women with irregular cycles, app predictions can be significantly inaccurate — use LH kits instead.
None of these findings are definitive diagnoses, but all warrant a conversation with your doctor rather than continued trying-in-the-dark.
References
Wilcox AJ et al. Timing of sexual intercourse in relation to ovulation. N Engl J Med. 1995;333(23):1517–21
Stanford JB & Dunson DB. Effects of sexual intercourse patterns in time to pregnancy studies. Am J Epidemiol. 2007
NICE Guideline CG156: Fertility Problems: Assessment and Treatment (2023)
Bull JR et al. Real-world menstrual cycle characteristics of women — large prospective observational study. NPJ Digit Med. 2019