How accurate is an LMP-based conception date?

It’s an estimate. Ovulation timing varies by several days even in regular cycles, and many people don’t recall their LMP precisely. First-trimester ultrasound dating is the most accurate method per ACOG.

Why is the fertile window six days long?

Sperm can survive in the female reproductive tract for up to five days, and the egg is viable for about 24 hours. So pregnancy is biologically possible from about five days before ovulation through one day after.

Does the cycle length really matter?

Yes — most of the variation between people is the time from period to ovulation. The 14-day luteal phase (ovulation to next period) is fairly stable, so we offset ovulation by cycle length − 14.

Why is the EDD-based estimate 266 days rather than 280?

The 280-day pregnancy count is measured from LMP. Conception happens about 14 days later, so from conception to delivery is 280 − 14 = 266 days.

Is my data stored anywhere?

No. All math runs in your browser; nothing is sent to a server.

Conception Calculator

Written by Golam Rabbani, Founder & Lead Engineer — Last updated 2026-05-01

This interactive tool requires JavaScript. Read the formula and worked example below; you can compute the result by hand.

How to use this conception calculator

Pick whether you’re estimating from your last menstrual period (LMP) or from a known due date.
Enter that date in the date picker.
If you picked LMP, adjust “average cycle length” away from 28 days if your cycle is shorter or longer.
Press “Estimate conception” to see the conception date, fertile window, and estimated due date.
Use Copy to save the dates or Reset to clear the form.

About this conception calculator

The conception calculator estimates the date of conception from either the first day of your last menstrual period (LMP) or from a known estimated due date (EDD). It uses the dating math embedded in ACOG’s pregnancy dating guidance (Committee Opinion 700, 2017): pregnancy is counted in gestational weeks from the LMP, ovulation in a 28-day cycle is on day 14, and full term is reached at 280 days from LMP (266 days from conception).

When you provide the LMP, the tool offsets by your cycle length: ovulation day = cycle length − 14, so a 32-day cycle puts ovulation on day 18 and a 26-day cycle on day 12. The fertile window is shown as 5 days before through 1 day after the estimated ovulation date, matching the Wilcox et al. (NEJM 1995) sperm-viability window. When you provide the due date, conception is estimated as EDD − 266 days regardless of cycle length.

Worked example: LMP on January 1 with a 28-day cycle ⇒ ovulation ≈ January 15, fertile window January 10 – January 16, EDD October 8.

This is date math only; actual ovulation varies day to day and the calculator is not a substitute for clinical confirmation of conception or due date by a qualified clinician. This calculator is for general educational purposes only and is not medical advice — consult a licensed healthcare professional for personal guidance.

FAQ

How accurate is an LMP-based conception date?: It’s an estimate. Ovulation timing varies by several days even in regular cycles, and many people don’t recall their LMP precisely. First-trimester ultrasound dating is the most accurate method per ACOG.
Why is the fertile window six days long?: Sperm can survive in the female reproductive tract for up to five days, and the egg is viable for about 24 hours. So pregnancy is biologically possible from about five days before ovulation through one day after.
Does the cycle length really matter?: Yes — most of the variation between people is the time from period to ovulation. The 14-day luteal phase (ovulation to next period) is fairly stable, so we offset ovulation by cycle length − 14.
Why is the EDD-based estimate 266 days rather than 280?: The 280-day pregnancy count is measured from LMP. Conception happens about 14 days later, so from conception to delivery is 280 − 14 = 266 days.
Is my data stored anywhere?: No. All math runs in your browser; nothing is sent to a server.