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Health

Conception Calculator

Estimate your conception date from a due date or last menstrual period (LMP). Find out when you likely conceived.

What is Conception Calculator?

The conception calculator estimates the date of conception either by working backward from a known due date, or forward from the first day of the last menstrual period (LMP). It also shows gestational age and, when starting from LMP, the estimated due date.

How to use

  1. 1 Choose your calculation method: 'From Due Date' or 'From Last Menstrual Period'.
  2. 2 Enter either your due date or the first day of your last period.
  3. 3 Click Calculate to see your results.
  4. 4 If calculating from LMP, you'll also see your estimated due date.
  5. 5 Gestational age is calculated from today's date if the pregnancy is ongoing.

Formula

From due date: Conception = Due date − 266 days (38 weeks). From LMP: Conception = LMP + 14 days. Due date from LMP = LMP + 280 days (40 weeks).

Example calculation

If your due date is March 15, 2025: Conception ≈ June 23, 2024 (March 15 − 266 days). If LMP was June 1, 2024: Conception ≈ June 15, 2024, and due date ≈ March 8, 2025.

Frequently asked questions

How accurate is the conception date estimate?

The estimate is accurate to within a few days for women with regular 28-day cycles. Conception actually occurs at ovulation, which averages 14 days after LMP but varies person to person.

What is the difference between conception date and implantation date?

Conception (fertilization) typically occurs within 24 hours of ovulation. Implantation — when the embryo attaches to the uterine wall — happens about 6–10 days later.

Why is pregnancy measured from LMP, not conception?

Ovulation and fertilization are difficult to pinpoint, but the first day of the last period is usually known. Using LMP as the start gives a standardized 40-week gestational calendar.

Can an ultrasound change the estimated conception date?

Yes. Early ultrasound (before 14 weeks) is more accurate than LMP-based dating and may shift the estimated due date and conception window by several days.

What does gestational age mean?

Gestational age measures the pregnancy from the first day of the LMP, not from conception. A full-term pregnancy is 40 weeks of gestational age (38 weeks from conception).