How does the female cycle work?
Ovulation, the fertile window and the biology behind it
The female menstrual cycle is divided into two phases: follicular phase (from the first day of menstruation until ovulation) and luteal phase (from ovulation until the start of the next menstruation). For most women, the luteal phase is relatively constant at around 14 days. The follicular phase, however, can vary from cycle to cycle, which is why ovulation occurs correspondingly earlier or later in shorter or longer cycles.
The formula for ovulation is therefore: ovulation ≈ cycle start + (cycle length − 14) days. In a 28-day cycle, ovulation falls on day 14; in a 35-day cycle, on day 21. From this ovulation date, our calculator determines the fertile window: 5 days before ovulation (because sperm can survive in the body for up to 5 days) until 1 day after ovulation (because the egg remains fertilizable for 12–24 hours).
For couples trying to conceive, the days of the fertile window – especially the 2–3 days around ovulation – are statistically the most favorable time for intercourse intended to result in conception. Current studies show that the highest probability of conception occurs on the day of ovulation and the day before. Regular intercourse every 2–3 days throughout the cycle can also be effective.
Important: the ovulation calculator provides a calculated estimate. Cycle variations caused by stress, illness, travel or hormonal changes can shift the actual ovulation date considerably. For reliable ovulation determination, gynecologists additionally recommend LH rapid tests, basal body temperature measurements or direct follicular ultrasound monitoring.