Reverse Due Date Calculator

A reverse due date calculator is a conception calculator that works from the due date and answers "when did I conceive?" It runs the pregnancy math backwards: you enter the estimated due date and it returns the date conception most likely happened, the first day of the last menstrual period, and every milestone in between. With a due date of December 6, 2026, conception lands on March 15, 2026 and the last period began on March 1, 2026.

Two sister tools cover the opposite directions. The Pregnancy Due Date Calculator goes forward from a last period or a known conception date to the due date, and the Gestational Age Calculator tells you how many weeks and days along a pregnancy is right now. Use this page when the due date is the one number you already have and everything else needs to be worked out from it.

How the reverse calculation works

Pregnancy is dated from the first day of the last menstrual period (LMP), not from conception, because that date is far easier to pin down. A standard due date is the LMP plus 280 days, which is 40 weeks. Ovulation, and therefore conception, happens roughly 14 days after the LMP in a textbook 28-day cycle. Reverse both steps and you get the two formulas this calculator uses:

The second line is the one that matters most. Because 280 minus 14 is 266, the estimated conception date sits 38 weeks before the due date. Around it the calculator draws a window of three days either side, a convention to show that ovulation is not a perfectly scheduled event: an NIH study found the fertile window can be highly unpredictable even in regular cycles.

Worked example: a due date of December 6, 2026

Take a due date of December 6, 2026 with an average cycle of 28 days. Step by step:

1. LMP: December 6, 2026 minus 280 days is March 1, 2026, a Sunday. That is week 0 of the pregnancy.
2. Ovulation and conception: 14 days after the period started (cycle day 15, counting March 1 as day 1), which is December 6 minus 266 days: March 15, 2026.
3. Conception window: three days either side gives March 12 to March 18, 2026.
4. Fertile window: pregnancies follow intercourse in the six days ending on ovulation day, so March 10 through March 15, 2026.
5. Implantation: 6 to 12 days after ovulation, most often 8 to 10 days, so March 21 to March 27, 2026.
6. First missed period: LMP plus the cycle length, which is March 29, 2026. The FDA notes that 10 to 20 of every 100 pregnant women still test negative on this day and suggests testing 1 to 2 weeks after a missed period for the most reliable result.

Read as a sentence: a baby due on December 6, 2026 was most likely conceived in the middle of March 2026, and a home test could start to pick up the pregnancy around the end of that month.

Conception date by due date

Every row below uses a 28-day cycle, so the LMP is the due date minus 280 days and conception is the due date minus 266 days. The window column is the three days either side of the estimate.

Notice the constant offset: conception is always exactly two weeks after the LMP row and exactly 38 weeks before the due date. Shift a due date by one day and both other dates shift by the same day.

Why cycle length moves the period, not the conception date

This is the part that trips people up. A 40-week due date assumes a 28-day cycle with ovulation 14 days after the period starts (cycle day 15, counting the first day of bleeding as day 1) and 14 days before the next one. If your cycles run longer, you did not conceive later, you ovulated later within a longer cycle, so the last period must have started earlier. The conception estimate stays exactly where it was. Here is the same December 6, 2026 due date across six cycle lengths:

Between the shortest and longest row the LMP moves eleven days while the conception date does not move at all. If the only thing you want from this page is the conception date, the cycle slider changes nothing; if you want the period date, it changes a lot.

A second worked example: a 32-day cycle

Now take a due date of August 20, 2027 for someone whose cycles run 32 days. Conception is still the due date minus 266 days, which is November 27, 2026, with a window of November 24 to November 30, 2026. The fertile window runs November 22 to November 27, and implantation would have fallen between December 3 and December 9, 2026. The LMP, though, is the due date minus 284 days, or November 9, 2026, four days earlier than the textbook answer of November 13. The first missed period would have been around December 11, 2026, later than a 28-day cycle would suggest, so a positive home test may also come later.

Key pregnancy dates from a due date

Once the LMP is known, every gestational milestone is just an offset from it. This table uses the December 6, 2026 due date and the March 1, 2026 LMP from the first example.

The calculator rebuilds this whole table for whatever due date you type, and it also marks where today falls on the timeline. On September 6, 2026 this example pregnancy would be 27 weeks 0 days along, about 68% of the way through the 40 weeks, with 91 days to go.

The implantation window and when a test turns positive

Conception is not the same as a positive test. After fertilization the embryo travels down the fallopian tube for several days and implants in the uterine lining. In an NIH study of 221 women, hCG, the hormone every pregnancy test looks for, first appeared in urine 6 to 12 days after ovulation, and 84% of the pregnancies that lasted at least six weeks implanted on day 8, 9 or 10. The FDA explains that hCG appears shortly after the embryo attaches to the wall of the uterus, so testing too early can give a negative result on a pregnancy that is really there. In the December 6, 2026 example, implantation falls between March 21 and March 27 and the first missed period is March 29; the FDA suggests testing 1 to 2 weeks after a missed period for the most reliable result.

IVF and IUI pregnancies

Assisted pregnancies are dated more precisely than natural ones, because the clinic knows exactly when fertilization happened. ACOG Committee Opinion 700 says the due date of an IVF pregnancy should be assigned from the embryo age and the transfer date: 261 days after a day-5 (blastocyst) transfer and 263 days after a day-3 transfer. Fertilization is therefore still the due date minus 266 days, the same anchor as natural conception. For an IUI cycle, treat the insemination date as the conception date, since insemination is timed to ovulation. Switch the calculator to the matching mode and it labels the output accordingly rather than showing a fertile window that does not apply.

How to use this calculator

1. Enter the due date. Use the date your provider is actually working from. If an early ultrasound moved it, use the ultrasound date rather than the original one.
2. Pick how the pregnancy started. Natural or IUI keeps the cycle-based view; the two IVF modes add the transfer date and drop the fertile window.
3. Set your average cycle length. Only needed in natural mode, and only to place the last period and the missed period correctly.
4. Read the headline date. The big date is the most likely conception day; the window under it is the honest range.
5. Scroll the timeline. The key-dates table gives you the last period, the trimester boundaries, the anatomy scan window, and the term dates all at once.

Everything recalculates the moment you change an input, and nothing is sent anywhere: the arithmetic runs in your browser.

Who this calculator is for

• Expectant parents who were handed a due date at a first scan and want to know which weekend in which month the pregnancy actually started.
• People with irregular cycles who cannot remember an LMP but do have a due date from an ultrasound.
• Anyone rebuilding a timeline for a pregnancy app, a baby book, or a family record where the due date is the only date written down.
• People planning ahead who want to know when they would need to conceive to hit a particular due date, since the same arithmetic works in either direction.
• Curious readers checking how a conception estimate is built before trusting one they saw somewhere else.

Key terms explained

• LMP: the first day of the last menstrual period. It is cycle day 1 and the zero point of gestational dating (0 weeks 0 days).
• Naegele's rule: the classic formula that sets the due date at LMP plus 280 days, the rule this page runs in reverse.
• Gestational age: the age of a pregnancy counted from the LMP, always about two weeks more than the age counted from conception.
• Fetal or conceptional age: the age counted from conception, which is what people usually mean casually and which is why the two numbers never match.
• Fertile window: the six days ending on the day of ovulation, when intercourse can result in pregnancy (Wilcox et al., 1995).
• Implantation: the embryo attaching to the uterine lining, 6 to 12 days after ovulation and most often 8 to 10 (Wilcox et al., 1999), after which hCG becomes detectable.
• Term categories (ACOG Committee Opinion 579): early term is 37 weeks 0 days through 38 weeks 6 days, full term is 39 weeks 0 days through 40 weeks 6 days, late term is 41 weeks 0 days through 41 weeks 6 days, and post-term is 42 weeks 0 days or more.

What changes the result the most

• Where the due date came from. A first-trimester ultrasound date is far more reliable than a remembered LMP, so the whole reverse timeline inherits that quality.
• Ovulation variability. Even in regular cycles ovulation can move a few days, which is exactly what the plus or minus three-day window represents.
• Cycle length. Moves the LMP and the missed-period date by the difference from 28 days, but leaves the conception date untouched.
• Whether the due date was ever revised. If a scan moved the due date by a week, every date on this page moves by that same week.
• Multiples. Twins and higher-order pregnancies often follow a different delivery plan, so the 40-week arithmetic fits them less well.

Tips for a more reliable answer

• Use the due date currently on your chart, not the first one you were given, if the two differ.
• Read the answer as a window. Saying "conception was in the middle of March" is defensible; naming one specific day is not.
• If you tracked ovulation with basal body temperature or an LH test that cycle, trust your own record over this estimate.
• If you know the LMP but not the due date, run the numbers forward instead with the Pregnancy Due Date Calculator, which also adjusts for cycle length.
• Save the key-dates table before an appointment. Having the anatomy-scan and term dates on hand makes scheduling conversations much faster.

Limitations and assumptions

• It assumes the due date is a standard 40-week date. A date set for a planned induction or cesarean is usually earlier, and would produce a conception estimate that is too early.
• It assumes ovulation happened 14 days before the next period would have been due, the usual convention. In real cycles the luteal phase varies by a few days.
• It cannot model anovulatory cycles, irregular bleeding, or breakthrough bleeding mistaken for a period.
• It does not adjust for twins or higher-order multiples.
• It is not evidence of when a pregnancy began for any legal, custody or paternity purpose, and it is not a substitute for prenatal care.

How it compares to related calculators

This page answers "when did I conceive, given this due date?" If your question points a different way, another tool fits better:

• To go forward from a last period or a known conception date to the due date, use the Pregnancy Due Date Calculator.
• To find out how many weeks and days along a pregnancy is today, use the Gestational Age Calculator.
• To count the exact days between any two of these dates, use the Date Calculator.
• To turn a birth date into an exact age in years, months and days later on, use the Age Calculator.

Sources

• American College of Obstetricians and Gynecologists (ACOG): Committee Opinion 700, Methods for Estimating the Due Date (reaffirmed 2025: LMP + 280 days; IVF due date 261 days after a day-5 and 263 days after a day-3 transfer; redating thresholds).
• ACOG: How Your Fetus Grows During Pregnancy (pregnancy is assumed to start 2 weeks after the first day of the LMP; trimesters; about 1 in 20 women give birth on the due date) and Committee Opinion 579, Definition of Term Pregnancy.
• Wilcox AJ, Weinberg CR, Baird DD (1995): Timing of sexual intercourse in relation to ovulation, New England Journal of Medicine (six-day fertile window ending on ovulation day).
• Wilcox AJ, Baird DD, Weinberg CR (1999): Time of implantation of the conceptus and loss of pregnancy, New England Journal of Medicine (implantation 6 to 12 days after ovulation, most often 8 to 10).
• Wilcox AJ, Dunson D, Baird DD (2000): The timing of the "fertile window" in the menstrual cycle, BMJ (ovulation timing varies widely, even in regular cycles).
• Jukic AM, et al. (2013): Length of human pregnancy and contributors to its natural variation, Human Reproduction (NIH study).
• U.S. Food and Drug Administration (FDA): Pregnancy (hCG) home tests (testing 1 to 2 weeks after a missed period; 10 to 20 of 100 pregnant women test negative on the first day of a missed period).
• Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD, NIH): Pregnancy: condition information.
• NICHD (NIH): Pre-pregnancy care and prenatal care.
• MedlinePlus, U.S. National Library of Medicine (NIH): Pregnancy.
• MedlinePlus (NIH): Prenatal care.