Which should I use: the reverse due date calculator or the pregnancy due date calculator?

Use the pregnancy due date calculator when you know the first day of the last menstrual period or a conception date and want an estimated due date, current week, and trimester dates. Use the reverse due date calculator when you already have a due date and want to work backward to the estimated last menstrual period, conception window, trimester boundaries, milestones, and appointment prompts.

Why do the two calculators use 280 days and 266 days?

Obstetric dating conventionally counts pregnancy as about 40 weeks, or 280 days, from the first day of the last menstrual period, which is why the pregnancy due date calculator adds 280 days in LMP mode and the reverse calculator subtracts 280 days from a due date. Conception is typically estimated about two weeks after LMP in a 28-day cycle, so the pregnancy calculator adds 266 days from a conception date and the reverse calculator places conception 14 days after its estimated LMP.

Can these calculators assign an official due date?

No. Both pages state their results are planning estimates, not medical advice. A clinician may use early ultrasound, cycle history, embryo transfer information, or other medical details to assign the official estimated due date, and births commonly occur before or after the estimate. Use the provider's date for appointments, screening windows, and medical decisions.

Reverse Due Date Calculator vs Pregnancy Due Date Calculator

The reverse due date calculator and the pregnancy due date calculator are the two ends of the same 40-week dating convention. The pregnancy due date calculator moves forward: enter the first day of the last menstrual period or a conception date, and it estimates the due date, current week, and trimester dates. The reverse due date calculator moves backward: enter a due date, and it estimates the last menstrual period, likely conception date, trimester boundaries, milestones, and appointment windows. If you know one end of the pregnancy calendar, the other tool finds the rest — the genuine choice is which date you start from.

Both pages are careful about boundaries: they are planning and education tools, not medical advice, and neither can prove a conception day, settle paternity, or replace the due date a clinician assigns from ultrasound or other dating methods.

What each calculator does

The pregnancy due date calculator has two modes. In last menstrual period mode it adds 280 calendar days:

$\text{due date} = \text{LMP date} + 280\text{ days}$

In conception mode it adds 266 days:

$\text{due date} = \text{conception date} + 266\text{ days}$

For the worked example, a first day of last menstrual period of January 1, 2026 gives an estimated due date of October 8, 2026, with first trimester ending March 26, 2026, second trimester ending July 2, 2026, and the full term date at September 17, 2026. A conception date of January 15, 2026 also gives October 8, 2026.

The reverse due date calculator subtracts 280 days from the due date for the estimated LMP, then adds 14 days for estimated conception:

$\text{estimated LMP} = \text{due date} - 280\ \text{days}$ $\text{estimated conception} = \text{estimated LMP} + 14\ \text{days}$

For the worked example, entering the due date October 8, 2026 returns January 1, 2026 as the estimated LMP and January 15, 2026 as the estimated conception date, with first trimester ending April 2, 2026, second trimester ending July 2, 2026, and third trimester starting July 9, 2026.

Side-by-side comparison

Feature	Pregnancy Due Date calculator	Reverse Due Date calculator
Direction	Forward: LMP or conception → due date	Backward: due date → LMP and conception
Core input	First day of LMP, or conception date	A single due date
Key formulas	LMP + 280 days; conception + 266 days	Due date − 280 days; LMP + 14 days
Worked example	LMP Jan 1, 2026 → due Oct 8, 2026	Due Oct 8, 2026 → LMP Jan 1, 2026, conception Jan 15, 2026
Extra outputs	Current week, trimester dates, full term date	Trimester boundaries, milestones, appointment prompts
Shared boundary	Planning estimate, not medical advice or clinician EDD	Planning estimate, not proof of conception or medical dating

Verdict and limitations

Use the pregnancy due date calculator when you know the period or conception date that started the clock and want the due date and timeline; use the reverse due date calculator when a due date is already set and you want to see what it implies for cycle timing, milestones, travel, or appointment windows. Both tools assume standard gestational dating: a 28-day cycle with ovulation near day 14, which irregular cycles and assisted reproduction may not follow. Ultrasound in early pregnancy can revise dating, and the calculators use plain Gregorian calendar days, so leap days and real month boundaries are preserved but no time-of-day or time zone is implied. Treat every date as a planning prompt for a healthcare professional, not a promise or a deadline.

Reverse Due Date vs Pregnancy Due Date