Reverse Due Date Calculator

You know your estimated due date but want to pinpoint when conception happened. A reverse due date calculator answers exactly that question – it works backward from your expected delivery date to reveal the most likely conception date and the start of your last menstrual period (LMP).

How a reverse due date calculator works

Pregnancy dating follows a well‑established timeline: the average human gestation lasts 280 days (or 40 weeks) from the first day of the last menstrual period. This convention, known as Naegele’s rule, assumes a 28‑day cycle with ovulation on day 14. Your healthcare provider calculates the due date by adding 280 days to the LMP.

The reverse calculator simply flips the direction. Starting from the due date, it subtracts 266 days to estimate conception – since conception typically occurs at ovulation, about 14 days after the LMP begins. Alternatively, subtracting 280 days from the due date gives the LMP.

Formula:

• LMP = Due Date − 280 days
• Conception date = LMP + 14 days (or Due Date − 266 days)

For example, if your due date is October 10, 2026:

• LMP would be around January 3, 2026
• Conception date would be approximately January 17, 2026

The calculator above performs these conversions instantly. Enter the due date you have from your doctor or ultrasound. The tool returns the estimated LMP, conception date, and how many weeks pregnant you are right now based on the entered due date. It requires no personal health data – only a calendar date.

How to calculate conception date from a due date manually

If you prefer to run the numbers yourself:

1. Start with your pregnancy’s due date.
2. Count back 266 days (or 38 weeks) to arrive at the estimated conception date.
3. To find the first day of your last period, count back an additional 14 days – a total of 280 days from the due date.

Keep in mind this works best for a textbook 28‑day cycle. Variations in cycle length or ovulation timing shift the result by a few days.

Limitations of reverse due date calculations

• Cycle irregularity – Not every woman ovulates on day 14. A 32‑day cycle might mean ovulation around day 18, pushing conception later.
• Sperm lifespan – Conception can occur up to five days after intercourse. The calculator gives the single most probable date, not a window.
• Pregnancy dating by ultrasound – First‑trimester ultrasound crown‑rump length is the gold standard for dating. Reverse calculations are approximations, not diagnostic.
• Multiple possible conception dates – If intercourse occurred on several days near ovulation, the calculator cannot distinguish the exact event.

Because of these natural variations, estimated conception dates from a reverse due date calculator carry an uncertainty of about ±3 to 5 days for a typical cycle.

This information is for educational purposes only and does not constitute medical advice. Always consult your healthcare provider for personalized pregnancy guidance.