Conception Calculator

How to use the conception calculator

1. Pick what you know: a due date, the first day of the last period before the pregnancy, or a birth date.
2. Enter that date. For a birth date, also enter the gestational age at birth in weeks and days, exactly as the hospital recorded it. A baby born on the due date is 40 weeks 0 days.
3. Enter the average cycle length (21 to 45 days). In due date mode it changes only the estimated last period shown, not the conception date.
4. Read the results: conception date, 7-day fertile window, estimated last period and due date.

How the conception date is estimated

Pregnancy is counted from the last menstrual period (LMP), not from conception. According to StatPearls, an average pregnancy lasts 280 days from the first day of the LMP, or 266 days after conception, and the rule assumes a 28-day cycle with ovulation on day 14. The calculator treats conception as the ovulation day and follows the same conventions as our due date and ovulation calculators:

• From a due date: conception = due date − 266 days. Cycle length doesn't move this date, because a due date already reflects when ovulation happened. The estimated LMP is conception − (cycle length − 14) days.
• From a last period: conception = LMP + (cycle length − 14) days, assuming 14 days from ovulation to the next period. Due date = LMP + 280 days + (cycle length − 28).
• From a birth date: conception = birth date − gestational age at birth + 14 days. The due date shown is the original estimate (birth date − gestational age + 280 days).
• Fertile window: 5 days before to 1 day after the estimated conception date. The NICHD notes that conception can occur only in the 6 days before ovulation through the day of ovulation: sperm stay functional for 5 days, the egg for usually 8 to 12 hours. Our window adds one day after ovulation as a safety margin, because the ovulation date itself is only an estimate.

Conception calculator examples

1. Due date April 10, 2027

Estimated conception: July 18, 2026, with a fertile window from July 13 to July 19, 2026. With a 28-day cycle, the estimated last period is July 4, 2026. With a 32-day cycle, conception stays on July 18 and only the estimated last period moves, to June 30.

2. Last period September 1, 2026

With a 28-day cycle, conception is estimated on September 15, 2026 (window September 10 to 16), and the due date is June 8, 2027. With a 35-day cycle: conception September 22, 2026, due date June 15, 2027.

3. "When was I conceived?" Born March 15, 2026

A baby born at 37 weeks 3 days was conceived 18 days later than one born at 40 weeks on the same day.

What the dates can and can't tell you

• It is a range, not a moment. Sex on any day of the window can lead to a pregnancy, so the conception date is not the date of intercourse. Ovulation also shifts from cycle to cycle: the Office on Women's Health says the time from ovulation to the next period can be anywhere from 7 to 19 days.
• Dates can't settle paternity. When two possible dates are days or even a couple of weeks apart, a calendar estimate cannot tell them apart. Only DNA testing can establish who the biological father is.
• The birth method depends on the recorded gestational age. If that figure was an estimate, so is the result.
• Not covered: IVF and other assisted conception (the timing is known from the procedure; the due date calculator has IVF modes), irregular cycles, luteal phases other than 14 days (the ovulation calculator lets you change it) and corrections for twins.