The Fertility Window Calculator helps you calculate your ovulation period based on your menstrual cycle.
Using the Shettles method, this calculator also predicts the baby's sex based on the timing of intercourse relative to the ovulation day. This method relies on the different characteristics of X and Y sperm.
Disclaimer: Sex prediction is an estimate and not a medical guarantee.
Calculator information
📋 How to use this calculator
- Enter the first day of your last menstrual period (LMP). Use your calendar, not a guess.
- Enter your average cycle length in days (typically 21-35). Count from the first day of one period to the day before the next.
- Pick a target sex for the baby: boy or girl. This determines the recommended intercourse timing per the Shettles method.
- Click Calculate to view the estimated ovulation date and the fertile window from 5 days before through 1 day after ovulation.
- Check the fertility calendar: days highlighted green are the most fertile; days highlighted blue/pink suggest timing for the gender target.
- Note: results are statistical estimates. Scientific accuracy of Shettles for gender selection is only about 50-75%.
🧮 Ogino-Knaus Calendar Method + Shettles Method
Ovulation day = LMP + (cycle length - 14); fertile window = ovulation - 5 days through ovulation + 1 day
- LMP = first day of last menstrual period
- 14 = length of the luteal phase (relatively constant)
- Shettles: intercourse 2-5 days before ovulation tends toward girls (X-sperm survive longer); intercourse on the day of ovulation tends toward boys (Y-sperm swim faster)
The Shettles method (1960) has not been confirmed by randomized trials. Its academic validity is debated; treat it as a calendar guide, not a guarantee.
💡 Worked example: Woman with 28-day cycle, LMP May 1, 2026
Given:- LMP = May 1, 2026
- Cycle length = 28 days
- Target = baby girl
Steps:- Compute ovulation day: 28 - 14 = 14, so ovulation falls on cycle day 14 = May 14, 2026
- Fertile window: May 9 through May 15, 2026
- For a girl (Shettles): intercourse recommended May 9-11 (2-5 days before ovulation)
- Avoid intercourse May 13-14, which would increase the odds of a boy
Result: Optimal dates for a baby girl: May 9-11, 2026. Estimated ovulation: May 14, 2026.
❓ Frequently asked questions
How accurate is the calendar method for identifying the fertile window?
The Ogino-Knaus calendar is most accurate for women with regular cycles of 26-32 days. For irregular cycles its accuracy falls sharply, and it is better to use LH ovulation predictor strips or basal body temperature tracking. WHO data show the calendar method alone predicts ovulation correctly only about 60-75% of the time.
Can the Shettles method really select a baby's sex?
Shettles has been popular since the 1960s, but larger studies have not confirmed its claimed 75% accuracy. A 1995 New England Journal of Medicine study found no significant link between intercourse timing and the sex of the baby. The only medically validated method for sex selection is preimplantation genetic testing (PGT) during IVF, which in the US is legal and clinically available but expensive and typically used for medical indications.
What is the difference between the fertile window and the ovulation day?
Ovulation is when the egg is released from the ovary, and it survives only 12-24 hours. The fertile window is the roughly 6-day span (5 days before plus the day of ovulation) when intercourse can lead to pregnancy because sperm can live 3-5 days in the female reproductive tract. Intercourse 2-3 days before ovulation often results in higher conception rates than intercourse on the day of ovulation.
My cycles are irregular - can I still use this calculator?
If your cycles vary by more than 7 days, the calendar method is not reliable. Better options include LH ovulation predictor kits (sold at any US pharmacy for about $15-30 per pack), daily basal body temperature charting, and observing cervical mucus changes. Consult an OB-GYN if your cycles are persistently irregular.
Is this safe to use as a natural birth control method?
The calendar (rhythm) method has a typical-use failure rate of about 24% per year per CDC data - 1 in 4 couples relying on it become pregnant within a year. For more effective natural family planning, combine basal body temperature with cervical mucus monitoring (the symptothermal method). For maximum protection, add condoms on fertile days.
📚 Sources & references
Last updated: May 11, 2026