# Reverse Due Date Calculator

Work backward from your due date to estimate when conception occurred. Accounts for cycle length and ovulation timing for a more accurate result.

## What this calculates

If you already know your due date and want to figure out when conception likely happened, this calculator works backward through the math. It reverses Naegele's rule and adjusts for your cycle length to estimate the conception date and your fertile window.

## Inputs

- **Days Until Due Date** (days) — min 0, max 300 — Number of days from today until the expected due date. Enter 0 if the due date is today.
- **Average Cycle Length** (days) — min 20, max 45 — Your typical menstrual cycle length in days (default 28).

## Outputs

- **Estimated Conception** — formatted as text — How many days ago conception likely occurred
- **Conception Window** — formatted as text — Range of days when conception most likely happened
- **Last Menstrual Period** — formatted as text — Estimated first day of the last menstrual period
- **Current Gestational Age** — formatted as text — How far along the pregnancy is right now

## Details

Standard due date calculators start from the first day of your last menstrual period and add 280 days. This calculator does the opposite. Given a due date, it subtracts 280 days to find the LMP, then moves forward to the estimated ovulation day to pinpoint conception.

Conception typically happens on or near the day of ovulation, which is about 14 days before the start of the next period. For a 28-day cycle, that puts ovulation around day 14. For a 32-day cycle, ovulation shifts to around day 18. The fertile window stretches from about 5 days before ovulation through 1 day after, because sperm can survive 3 to 5 days in the reproductive tract.

Keep in mind that these are estimates. Even with regular cycles, ovulation can shift by a few days. First-trimester ultrasound remains the gold standard for dating a pregnancy. If you need precise dating for legal or medical reasons, consult your healthcare provider.

## Frequently Asked Questions

**Q: How does a reverse due date calculator work?**

A: It takes your known due date and subtracts 280 days (the standard pregnancy length from LMP) to find the estimated first day of your last period. Then it adds your estimated ovulation day (cycle length minus 14) to arrive at the likely conception date. The result is adjusted for your individual cycle length.

**Q: Why is there a conception window instead of an exact date?**

A: Sperm can survive in the reproductive tract for 3 to 5 days, and the egg is viable for about 12 to 24 hours after release. That means intercourse several days before ovulation can result in conception. The window accounts for this biological variability.

**Q: Can I use this for legal paternity purposes?**

A: This calculator provides an estimate, not a definitive answer. Actual conception timing can vary by several days even with regular cycles. For legal or paternity questions, DNA testing and first-trimester ultrasound dating are the accepted standards. Always consult a medical professional.

**Q: Does cycle length really change the conception date?**

A: Yes. The luteal phase (ovulation to period) is fairly constant at about 14 days, but the follicular phase (period to ovulation) varies with cycle length. A 35-day cycle means ovulation around day 21 instead of day 14, pushing the estimated conception date back by a full week compared to a 28-day cycle.

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Source: https://vastcalc.com/calculators/health/reverse-due-date
Category: Health & Fitness
Last updated: 2026-04-08