Due Date Calculator
Calculate your pregnancy due date and current week — enter your last period or conception date. See trimester, milestones and key scan dates. Free, no signup.
Due Date Calculator Tool
Pregnancy Progress
Key Milestone Dates
How to Use the Due Date Calculator
Choose LMP, Conception Date or IVF Transfer
If you know the first day of your last menstrual period, use the LMP tab; that is the conventional starting point. If you know the conception date, use the Conception Date tab. After IVF, use the IVF Transfer tab with the transfer date and the embryo's age, fresh or frozen.
Enter the date
Select the date using the date picker. In LMP mode, also adjust the cycle length if your cycle is not the default 28 days — the calculator adds or subtracts days accordingly. Results appear the moment you pick a date.
See your due date and current week
Your estimated due date is displayed prominently. Below it, the current pregnancy week, trimester, and days remaining are shown. The progress bar visualises your position in the 40-week journey, with trimester boundaries clearly marked.
Review key milestone dates
The milestone list shows the dates for your Week 8 first visit, Week 12 NT scan, Week 20 anatomy scan, the start of early term at Week 37, and other common appointments. Each milestone shows the calendar date and how many weeks away (or ago) it falls.
Pregnancy Timeline — Understanding Your 40 Weeks
How the due date is calculated
By convention the estimated due date is 280 days after the first day of the last menstrual period (LMP), which is 40 weeks. That is the convention ACOG states, and it is what this calculator counts.
It is often called Naegele's rule, but the calendar shortcut taught under that name, nine months and seven days on, is a different calculation. Months are not all the same length, so it lands on the same day as the 280-day count only some of the time. This calculator ran both over every start date from 2021 to 2024, one full leap-year cycle, moving a date that does not exist in the target month to that month's last day (31 May lands on the end of February). The shortcut matched on 44% of start dates and was otherwise 1 to 3 days later; it was never earlier. The results panel shows its date beside the 280-day one.
Franz Naegele's own wording, in his textbook for midwives, was to count three whole months back from the last period and add seven days, and he warned that the day of birth can only ever be foretold approximately (Lehrbuch der Geburtshülfe für die Hebammen, 1833 edition, §206). The passage of his 1812 book often cited for the rule (pp. 280 to 281) gives no day count: it quotes, from Haller's edition of Boerhaave's lectures, a claim that 99 births in 100 fall in the ninth month counted from a week after the last period.
ACOG notes that the 280-day convention assumes a 28-day cycle with ovulation on day 14. For a longer or shorter cycle this calculator extends that assumption, moving the date by the difference, so a 35-day cycle gives a date seven days later. That shift is the calculator's assumption, not an ACOG formula. In Conception Date mode it adds 266 days, the 280 minus the 14 days before ovulation.
The American College of Obstetricians and Gynecologists (ACOG) is frank about the weaknesses of the LMP method. It says the method does not account for inaccurate recall, irregular cycles or variable ovulation, and that approximately one half of women accurately recall their LMP. In one study it cites, 40% of the women randomized to a first-trimester scan had their due date adjusted because the scan and the LMP date differed by more than 5 days.
"Ultrasound measurement of the embryo or fetus in the first trimester (up to and including 13 6/7 weeks of gestation) is the most accurate method to establish or confirm gestational age."
ACOG Committee Opinion 700, Methods for Estimating the Due Date (2017)When a scan changes the date
Up to 13 weeks and 6 days, dating by the crown-rump length of the fetus is accurate to within ±5 to 7 days, according to ACOG. If a scan before 14 weeks differs from the LMP date by more than 7 days, the due date should be changed to match the scan; before 9 weeks, a difference of more than 5 days is reason enough. Later scans are less precise: ±7 to 10 days between 14 and 21 weeks, when ACOG advises re-dating if the two differ by more than 7 days at 14 to 15 weeks or more than 10 days at 16 to 21 weeks, and ±10 to 14 days at 22 to 27 weeks. Once set, the date should change only in rare circumstances.
For IVF the date comes from the embryo, not the calendar. ACOG says the due date should be assigned from "the age of the embryo and the date of transfer", and gives 261 days from transfer for a day-5 embryo and 263 days for a day-3 embryo. The IVF Transfer tab counts exactly that. It works the same for a frozen transfer, where the egg retrieval may have been months or years before; counting from the retrieval date would put the due date far too early.
How this calculator counts weeks and trimesters
Weeks are counted from the LMP, from the date two weeks before conception, or for IVF from the transfer date less the embryo's age and two weeks, and shown as "Week N + D days". ACOG's dating guidance puts the first trimester up to and including 13 weeks 6 days and the second from 14 weeks 0 days to 27 weeks 6 days, so this tool counts week 28 onwards as the third. From 37 weeks it also shows the term category ACOG uses. The milestone list shows common appointment weeks, such as the first-trimester scan and the mid-pregnancy anatomy scan, as calendar dates. Your own clinic may schedule them differently.
What ACOG says about dating a pregnancy
By convention the due date is 280 days after the first day of the last period, which assumes a 28-day cycle and ovulation on day 14.
ACOG reports that approximately one half of women accurately recall the date of their last period.
In a study ACOG cites, 40% of the women randomized to a first-trimester scan had their due date adjusted because the scan and the LMP date differed by more than 5 days.
Up to 13 weeks and 6 days, crown-rump length dating is accurate to ±5 to 7 days.
If a scan before 14 weeks differs from the LMP date by more than 7 days, ACOG says the scan date should be used.
Scan dating gets less precise as pregnancy goes on: ±10 to 14 days between 22 and 27 weeks.
For IVF, the due date is 261 days after a day-5 embryo transfer, or 263 days after a day-3 transfer.
Conception Date mode adds 266 days: the 280-day convention minus the 14 days before ovulation.
In a UK study of 34,249 singleton pregnancies with certain period dates, 49.5% delivered within 7 days of the period-based date and 64.1% within 10 days; dated by ultrasound, 55.2% and 70.3% (Mongelli, Wilcox and Gardosi, 1996).
ACOG says that once the due date is set, later changes should be rare, and discussed with the patient.
Frequently Asked Questions
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The calculator adds 280 days (40 weeks) to the first day of your last period, the convention ACOG describes. If your cycle length is not 28 days, it assumes ovulation moves with it and shifts the date by one day for each day above or below 28; that shift is the calculator's assumption. In Conception Date mode, due date = conception date + 266 days (38 weeks).
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In everyday use, Naegele's rule means adding nine months and seven days to the first day of the last period. Franz Naegele's own wording, in his textbook for midwives, was to count three whole months back and add seven days. It is not the same as adding 280 days: because months differ in length, it gives the same day or up to three days later. The 280-day count is the modern convention ACOG describes, and it is the one this calculator uses; the results show the calendar rule's date beside it for comparison.
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It is an estimate, and births spread around it. In a UK study of 34,249 singleton pregnancies with certain period dates (Mongelli, Wilcox and Gardosi, 1996), 49.5% delivered within 7 days of the period-based date and 64.1% within 10 days; dated by ultrasound, the shares were 55.2% and 70.3%. A first-trimester scan is more accurate than the LMP date: ACOG puts crown-rump length dating at ±5 to 7 days up to 13 weeks and 6 days, and it may move your due date. The date your clinician sets from the scan is the one to use.
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Use the cycle length selector to choose your average cycle length (21–35 days). The calculator adjusts the due date accordingly. However, if your cycles are very irregular, the LMP method is less reliable — ultrasound dating at your first prenatal visit will give a more accurate result.
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Yes. ACOG advises that if a scan before 14 weeks differs from the LMP date by more than 7 days, the due date should be changed to match the scan; before 9 weeks, more than 5 days is enough. Later in pregnancy the thresholds are wider because scans are less precise. A revised date is normal. It means the scan is a better measure of how far along you are than a remembered date.
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ACOG says an IVF due date should come from the embryo's age and the transfer date: 261 days after transfer for a day-5 embryo and 263 days for a day-3 embryo. Use the IVF Transfer tab, enter the transfer date and pick the embryo's age. That works for fresh and frozen transfers alike; do not use the egg retrieval date, which after a frozen transfer can be months earlier. Your clinic's date is the one to use.
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A birth before 37 weeks 0 days is preterm. ACOG and the Society for Maternal-Fetal Medicine divide the weeks after that into early term (37 weeks 0 days to 38 weeks 6 days), full term (39 weeks 0 days to 40 weeks 6 days), late term (41 weeks 0 days to 41 weeks 6 days) and postterm (42 weeks 0 days and beyond) (ACOG Committee Opinion 579). A due date marks 40 weeks 0 days, inside full term rather than at its end. The calculator shows the category from 37 weeks.
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Going past the due date is common, because it marks the middle of the normal range rather than its end. As a pregnancy continues beyond 41 weeks your obstetrician will usually monitor you more closely and talk to you about induction. Follow the plan your own clinic gives you.
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Your obstetrician sets the date, usually once your dating has been confirmed by ultrasound. The week-by-week dates in this calculator show where a proposed date falls in your pregnancy, but they are not a booking.
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Method & sources
What this tool implements
- ACOG CO 700 (May 2017, reaffirmed 2025): "By convention, the EDD is 280 days after the first day of the LMP", a practice that "assumes a regular menstrual cycle of 28 days, with ovulation occurring on the 14th day"; first-trimester ultrasound "(up to and including 13 6/7 weeks of gestation) is the most accurate method"; second trimester "14 0/7 weeks to 27 6/7 weeks". The (cycle − 28) shift extends ACOG's assumption and the page labels it the calculator's own.
- IVF, ACOG CO 700: "the EDD for a pregnancy that resulted from in vitro fertilization should be assigned using the age of the embryo and the date of transfer … for a day-5 embryo, the EDD would be 261 days from the embryo replacement date … for a day-3 embryo would be 263 days". The IVF tab takes the transfer date and embryo age, so a frozen transfer is dated correctly.
- Re-dating thresholds as ACOG CO 700 gives them: more than 7 days before 14 0/7 weeks, more than 5 days before 9 0/7 weeks; "approximately one half of women accurately recall their LMP"; in one study "40% of the women randomized to receive first-trimester ultrasonography had their EDD adjusted because of a discrepancy of more than 5 days".
- ACOG CO 579 (Nov 2013, reaffirmed 2025): early term 37 0/7-38 6/7 weeks, full term 39 0/7-40 6/7, late term 41 0/7-41 6/7, postterm 42 0/7 and beyond. Shown from 37 weeks and stated in the FAQ.
- Mongelli, Wilcox & Gardosi 1996 (n = 34,249 singleton pregnancies with certain menstrual dates): delivery within ±7 days of the estimate in 49.5% by menstrual dates and 55.2% by ultrasound; ±10 days 64.1% and 70.3%.
- Earlier corrections (2026-09-30): dates computed in local time moved the due date a day across a DST change; IVF had been transfer + 266, five days late for a blastocyst; unsourced KKH birth totals, Baby Bonus, Medisave, fertility-rate, miscarriage, morning-sickness and GBS figures and "about 5% born on the due date" were removed.
Sources
- American College of Obstetricians and Gynecologists. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol 2017;129:e150-4. https://www.acog.org/clinical/clinical-guidance/committee-opinion…
- American College of Obstetricians and Gynecologists, Society for Maternal-Fetal Medicine. Committee Opinion No. 579: Definition of Term Pregnancy. Nov 2013. https://www.acog.org/clinical/clinical-guidance/committee-opinion…
- Mongelli M, Wilcox M, Gardosi J. Estimating the date of confinement: ultrasonographic biometry versus certain menstrual dates. Am J Obstet Gynecol 1996;174(1 Pt 1):278-81. https://doi.org/10.1016/s0002-9378(96)70408-8
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