The fertile window exists and has a date and time
Conception is only possible within a window of exactly 6 days per cycle: the 5 days before ovulation and ovulation day itself. This is the central finding of Wilcox, Weinberg, and Baird in the New England Journal of Medicine in 19951 — one of the most cited studies in reproductive medicine, with 221 healthy women and 625 cycles monitored by urinary estrogen and progesterone markers. After ovulation day, the window closes. The egg survives at most 12 to 24 hours. Sperm, however, can remain viable in the female reproductive tract for up to 5 days when cervical mucus conditions are favorable. This means that having sex before ovulation is just as strategic as on the exact day. Levvi displays this window directly on the cycle screen, calculated from the date of the last period and the average duration of previous cycles.
Real conception probability by cycle day
The Wilcox et al. (1995) study1 mapped conception probability per act of intercourse for each day relative to ovulation (day 0). These data were confirmed decades later by Faust et al. (2019)4 in a cohort of 225,596 cycles from fertility app users:
- Day -5 (5 days before ovulation): ~10% probability
- Day -4: ~14% probability
- Day -3: ~16% probability
- Day -2: ~27-33% probability (peak)
- Day -1 (the day before ovulation): ~30% probability
- Day 0 (ovulation day): ~15% probability
- Day +1 (after ovulation): probability close to zero
A counterintuitive detail: the probability on ovulation day itself (day 0) is lower than on the 2 preceding days. This happens because sperm needs hours to capacitate in the female tract. Arriving early pays off. Li, Wilcox, and Dunson (2015)3 calculated that a single random act of intercourse during a cycle has only a 25% chance of falling within the fertile window. This explains why couples who do not identify ovulation take longer to conceive — not because of infertility, but because of poor timing.
Ideal frequency: not every day, not once a week
The review by Clay, Zleczewski, and Stroie (2026)2 published in Translational Andrology and Urology evaluated the impact of ejaculatory frequency across four domains: volume and concentration, motility and viability, DNA fragmentation, and clinical fertility outcomes. Intercourse every 1 to 2 days during the fertile window optimizes all these parameters simultaneously. Lower frequency (prolonged abstinence) raises semen volume and total count but worsens progressive motility and increases sperm DNA fragmentation — two factors directly associated with fertilization failure. Very high frequency (daily for weeks) can reduce total concentration without additional motility benefit. For natural TTC, the ideal zone falls between 2 and 4 times per week throughout the cycle, with emphasis on the 5 days before ovulation.
The abstinence myth: saving up can backfire
One of the most common mistakes among TTC couples is deliberate abstinence: avoiding intercourse in the early cycle days to accumulate sperm and release everything during the fertile window. The logic seems reasonable, but data point in the other direction. Clay et al. (2026)2 show that abstinence exceeding 5 to 7 days leads to a measurable increase in sperm DNA fragmentation — a metric standard semen analysis often does not capture but which directly affects embryo quality and implantation rates. Older sperm accumulate oxidative damage. Abstinence of 2 to 3 days is sufficient to replenish concentration without compromising motility or genetic integrity. Levvi helps the couple visualize the full cycle, making any accumulation strategy unnecessary: proper timing replaces volume.
When knowing the ovulation date changes everything
The main bottleneck for couples without fertility problems is not sperm quality or uterine health: it is timing. Ovulation rarely falls on day 14 for every woman. Cycles of 24 to 35 days are normal, and ovulation can vary by up to 7 days within the same cycle in the same individual. Assuming day 14 is always correct wastes entire cycles of trying. Faust et al. (2019)4 confirmed in 225,596 real cycles that the conception probability curve by day relative to ovulation replicates the Wilcox et al. (1995) data with precision1 — but only when timing is calculated relative to actual ovulation, not a fixed calendar date. Levvi uses the partner's cycle history to estimate ovulation based on the individual's pattern, not a generic 28-day population average.
What to do after ovulation
A study by Steiner et al. (2014)5 published in Fertility and Sterility raised a controversial finding: frequent intercourse during the implantation window (5 to 9 days after ovulation) was associated with lower fecundability — a ratio of 0.62 compared to cycles without intercourse in that period. This finding is contested: Stanford et al. (2020) did not reproduce the result in a European study with 661 women. The current consensus is that there is no reason to avoid intercourse after ovulation, but also no documented benefit to intensifying it. The practical focus for TTC couples should be on the 5 days before ovulation.
Practical summary: what the science recommends
Based on Wilcox et al. (1995)1, Clay et al. (2026)2 and Li, Wilcox, and Dunson (2015)3, the most evidence-based guidance is:
- Identify the fertile window based on the partner's real cycle (not a fixed 28-day calendar)
- Have intercourse every 1 to 2 days in the 5 days before estimated ovulation (highest probability: days -2 and -1)
- Maintain regular frequency of 2 to 4 times per week throughout the entire cycle, not just during the fertile window
- Avoid abstinence of more than 5 to 7 days before the fertile window: the gain in volume does not compensate for the loss in motility and DNA integrity
- Do not obsess over timing: stress reduces sperm quality and libido, which defeats the very goal
Levvi automatically calculates the fertile window based on the partner's logged cycle data and sends notifications on the highest-probability days. This eliminates reliance on generic tables and turns cycle tracking into an objective tool for the couple.