How to use this tool
- Enter the first day of your last period.
- Set your usual cycle length, counted from the first day of one period to the first day of the next.
- Read your fertile window, likely ovulation day and the next three cycles.
How the dates are worked out
The second half of the cycle, from ovulation to the next period (the luteal phase), is fairly constant at around 14 days for most women. The first half varies more. So the calculator counts back 14 days from your next expected period to estimate ovulation, then marks the five days before it as fertile, because sperm can survive in the reproductive tract for up to five days while the egg lives for about a day.
A landmark study by Wilcox and colleagues found that almost all pregnancies came from intercourse in that six-day window, with the highest chances in the two days before ovulation.
Making the estimate more accurate
If your cycles vary by more than a few days, calendar estimates become less reliable. Urine LH (ovulation) test strips, cervical mucus changes and basal body temperature tracking can show when ovulation is really happening. A doctor can check for causes of irregular cycles such as polycystic ovary syndrome or thyroid problems.
Not reliable for contraception
Calendar-only "safe days" methods have a high failure rate in typical use. If you want to avoid pregnancy, speak to a clinician about reliable contraception.
Frequently asked questions
When am I most fertile?
In the two days before ovulation and on ovulation day. In a 28-day cycle that is roughly days 12 to 14.
How many days after my period do I ovulate?
It depends on cycle length. Ovulation is about 14 days before your next period, not 14 days after the last one.
Can I get pregnant outside the fertile window?
It's unlikely but possible, especially if your cycles are irregular.
When should I take a pregnancy test?
From the day your period is due. Testing earlier can give a false negative.
Sources and references
- ACOG: Fertility awareness-based methods of family planning
- NHS: Trying to get pregnant
- Wilcox AJ et al. Timing of sexual intercourse in relation to ovulation. NEJM 1995;333:1517–21