Reverse Due Date Calculator

Sometimes the date you know is the due date, not the period date that started the pregnancy clock. The Reverse Due Date Calculator works backward from that due date to estimate the last menstrual period, likely conception date, trimester boundaries, fetal-development milestones, and common prenatal appointment windows. It is for planning and education, not diagnosis.

What the calculator shows

Enter one due date. The calculator parses it as a calendar date, subtracts 280 days for the estimated last menstrual period (LMP), and treats that as gestational week 0. It then estimates conception 14 days after LMP, ends the first trimester at week 13, ends the second trimester at week 26, and starts the third trimester at week 27. It also lists development milestones from week 4 through week 40 and appointment prompts from week 8 through week 38.

If you know the LMP and want to move forward instead, use the gestational age calculator. If your question is focused on ovulation or possible conception timing, compare the pregnancy conception calculator. For non-pregnancy date intervals, the age calculator and time duration calculator are better fits.

Method and formulas

Obstetric dating commonly describes pregnancy as about 40 weeks from the first day of the last menstrual period. Conception is often estimated near two weeks after LMP in a 28-day cycle, although individual timing varies. The calculator uses these fixed relationships:

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

$\text{estimated conception} = \text{estimated LMP} + 14\ \text{days}$

$\text{first trimester end} = \text{estimated LMP} + 13 \times 7\ \text{days}$

$\text{second trimester end} = \text{estimated LMP} + 26 \times 7\ \text{days}$

$\text{third trimester start} = \text{estimated LMP} + 27 \times 7\ \text{days}$

Milestone and appointment rows use the same week-counting approach. For example, a week 24 milestone is placed at due date - (40 - 24) times 7 days, which is 112 days before the due date.

Worked example: due date October 8, 2026

Enter 2026-10-08 as the due date. The calculator subtracts 280 days and returns January 1, 2026 as the estimated LMP. It then adds 14 days and returns January 15, 2026 as the estimated conception date. The trimester dates follow directly from the LMP:

$\text{January 1, 2026} + 91\ \text{days} = \text{April 2, 2026}$

$\text{January 1, 2026} + 182\ \text{days} = \text{July 2, 2026}$

$\text{January 1, 2026} + 189\ \text{days} = \text{July 9, 2026}$

So the calculator reports First trimester end April 2, 2026, Second trimester end July 2, 2026, and Third trimester start July 9, 2026. Development milestones include week 20, anatomy scan ultrasound, on May 21, 2026, and week 40, due date, on October 8, 2026. Appointment prompts include week 24 glucose screening on June 18, 2026 and week 36 Group B strep test on September 10, 2026.

How to use the timeline

For pregnancy planning, the reverse view helps you see what a target due date implies for likely cycle timing, seasonal travel, parental leave, insurance enrollment, and prenatal appointments. For early pregnancy, it helps reconcile a clinician’s due date with the estimated LMP and conception timing. For records, keep the due date source clear: LMP-based dating, early ultrasound, IVF transfer, and clinician revision can produce different timelines.

The milestone list is intentionally broad. A heartbeat may be detected around week 6 in some contexts, movement may be felt around week 16 or later, and viability discussions around week 24 are medically complex. Treat the dates as prompts for questions, not promises that an event must occur on that day. Appointment rows work the same way: they place common screening or visit windows on the calendar so you can prepare questions, request time off, or compare the schedule your clinician gives you.

For reverse planning before pregnancy, think in windows rather than single dates. The estimated conception date is a midpoint implied by the due date, while fertile timing can span several days and may shift with cycle length. If a month is important because of work, travel, school terms, or insurance coverage, discuss timing with a healthcare professional instead of relying on one calculated day.

Edge cases, limitations, and common mistakes

Irregular cycles are the main limitation. If ovulation occurs much earlier or later than day 14, the estimated conception date shifts. Assisted reproduction has its own dating rules based on retrieval, fertilization, or embryo transfer timing. Ultrasound in early pregnancy can revise a due date when measurements differ from LMP expectations. Multiple pregnancies and medical conditions may also change monitoring and delivery planning.

Do not use this calculator to settle paternity questions or to make medical decisions about testing, medication, travel, or delivery timing. Do not assume a due date is a deadline; births commonly occur before or after it. Use the tool to organize questions for a healthcare professional and to understand how standard gestational dating works backward on a calendar.

Calendar and date boundaries

The calculator subtracts Gregorian calendar days from the due date, preserving leap days and real month and year boundaries. It uses date-only values, so time zone and daylight-saving transitions do not alter the milestones. Crossing a date boundary changes the displayed date by one day; it does not imply that conception occurred at midnight or at one exact instant.

Sources

• ACOG, How Your Fetus Grows During Pregnancy — patient guidance on fetal development by pregnancy stage.
• ACOG, Routine Tests During Pregnancy — timing context for common prenatal tests and screenings.
• CDC, About Pregnancy — public-health overview of pregnancy and prenatal care.