Calculateur de Date Prévue d'Accouchement

Votre date d'accouchement selon les dernières règles, la conception ou le transfert de FIV, avec semaines de grossesse, trimestres et calendrier

Calculez votre date d'accouchement

À partir de vos dernières règles, de la date de conception ou du transfert de FIV : date prévue, semaines de grossesse, trimestres et calendrier semaine par semaine

Une estimation, pas un avis médical. L'échographie de datation de votre médecin ou sage-femme prime sur tout calculateur. En cas de saignement, de douleur ou de toute autre inquiétude, contactez votre professionnel de santé.

Dernière mise à jour: octobre 5, 2026

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How a Due Date Is Calculated

There are three ways to date a pregnancy without a scan, and they all land on the same calendar if the inputs agree. The calculator uses whichever one you know:

From your last period

Naegele's rule: the first day of your last period plus 280 days (40 weeks). With a cycle longer or shorter than 28 days, ovulation moves, so the date moves by the same number of days.

From conception

If you know when you ovulated or conceived, add 266 days (38 weeks). This skips the assumption about your cycle.

From an IVF transfer

The embryo's age is known exactly: add 263 days to a day-3 transfer, or 261 days to a day-5 (blastocyst) transfer.

Whichever you use, weeks are counted from the first day of the last period, not from conception. That is why you are already about 2 weeks pregnant on the day you conceive, and 4 weeks pregnant around the time a period is missed.

Worked Examples

The same pregnancy dated three ways gives the same answer, and a longer or shorter cycle shifts it day for day:

You knowRuleDue dateEstimated conception
Last period January 1, 2026, 28-day cycleLMP + 280 daysOctober 8, 2026January 15, 2026
Last period January 1, 2026, 32-day cycleLMP + 280 + 4 daysOctober 12, 2026January 19, 2026
Last period January 1, 2026, 25-day cycleLMP + 280 - 3 daysOctober 5, 2026January 12, 2026
Conception January 15, 2026Conception + 266 daysOctober 8, 2026January 15, 2026
Day-5 embryo transferred January 20, 2026Transfer + 261 daysOctober 8, 2026January 15, 2026
Day-3 embryo transferred January 18, 2026Transfer + 263 daysOctober 8, 2026January 15, 2026

Trimesters and Term

StageGestational ageWhat it means
First trimester0 weeks to 13 weeks 6 daysOrgans form; most early screening happens near the end
Second trimester14 weeks 0 days to 27 weeks 6 daysAnatomy scan around 18 to 22 weeks, glucose test from 24 weeks
Third trimester28 weeks 0 days to birthGrowth, Group B strep test at 36 to 37 weeks
Early term37w0d to 38w6dACOG term category
Full term39w0d to 40w6dThe week before to the week after the due date
Late term41w0d to 41w6dClinicians usually discuss monitoring or induction
Post-term42w0d and laterBeyond the usual range

The term categories come from ACOG Committee Opinion 579. Babies born before 37 weeks are preterm.

Why the Date Is a Window, Not a Day

Only about 4% of babies are born on their due date. The date marks the middle of the likely range, and most births fall between 37 and 42 weeks. First pregnancies tend to run slightly longer on average, and natural variation in pregnancy length is several days either side even when the conception date is known.

If you are planning time off work or travel, plan around the full-term window (39 weeks 0 days to 40 weeks 6 days) rather than the single date, and keep the early-term weeks in mind.

When Your Doctor's Date Differs

A period-based date assumes ovulation 14 days before your next period, which is not true for everyone. That is why ACOG, with the AIUM and SMFM, recommends confirming the due date with an ultrasound in the first trimester, when the crown-rump length is the most accurate measure of gestational age. If the scan and your period date differ by more than about a week, your clinician will usually go with the scan, and once your due date is set it is not normally changed again. Use this calculator to plan and to understand the numbers, and use the date your doctor or midwife gives you for anything medical.

Comment nous calculons cela

Méthode
Naegele's rule adjusted for cycle length; conception and IVF dating; gestational age counted from the last period
Formule
From the last period: due date = LMP + 280 days + (cycle length - 28). From conception: due date = conception + 266 days. From IVF transfer: due date = transfer + 266 - embryo age (day 3: +263, day 5: +261). Adjusted LMP = due date - 280; gestational age on a date = date - adjusted LMP, in weeks + days; estimated conception = due date - 266. Trimesters: 1st 0w0d-13w6d, 2nd 14w0d-27w6d, 3rd from 28w0d. Term (ACOG): early 37w0d-38w6d, full 39w0d-40w6d, late 41w0d-41w6d, post-term 42w0d+.
Source
ACOG Committee Opinion No. 700, Methods for Estimating the Due Date (2017, with AIUM and SMFM). ACOG Committee Opinion No. 579, Definition of Term Pregnancy (2013). Test windows: ACOG Practice Bulletin No. 226 (aneuploidy screening: cell-free DNA from 10 weeks, first-trimester screening 11w0d-13w6d, quad screen 15-22 weeks), ACOG guidance on the 18-22 week ultrasound, Practice Bulletin No. 190 (glucose screening 24-28 weeks), Committee Opinion No. 797 (Group B strep culture 36w0d-37w6d), CDC/ACOG Tdap guidance (27-36 weeks).
Limites
An estimate, not a diagnosis. Naegele's rule assumes ovulation 14 days before the next period; irregular cycles, a misremembered date or recent hormonal contraception make the period-based date less reliable. A first-trimester ultrasound by your clinician takes precedence (ACOG re-dates when it differs by more than 5 days before 9 weeks, or more than 7 days from 9w0d to 13w6d). Only about 4% of babies arrive on the due date. Test timings are typical US windows; your clinician may schedule them differently.
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Foire Aux Questions

The standard method, Naegele's rule, adds 280 days (40 weeks) to the first day of your last menstrual period. That assumes a 28-day cycle with ovulation on day 14, so this calculator moves the date one day later for every day your cycle is longer than 28 days, and one day earlier for every day it is shorter. From a known conception date the pregnancy lasts about 266 days (38 weeks); from an IVF transfer it is 266 days minus the embryo's age, so 263 days for a day-3 embryo and 261 for a day-5 blastocyst.

It is an estimate of the middle of the likely window, not a delivery appointment. Only about 4% of babies are born on their due date. ACOG calls 39 weeks 0 days through 40 weeks 6 days "full term", and most births fall somewhere between 37 and 42 weeks. Dates from IVF are the most precise because the embryo's age is known exactly; dates from a remembered period are the least precise.

Pregnancy is counted from the first day of the last period, not from conception, because the period date is usually known and ovulation is not. In a 28-day cycle, ovulation and conception happen about two weeks after the period starts, so a pregnancy is already about 2 weeks 0 days along at conception and 4 weeks along around the time of a missed period.

Follow the ultrasound. ACOG recommends that an ultrasound measurement in the first trimester (crown-rump length, up to 13 weeks 6 days) is the most accurate way to confirm or set the due date, and that the date is changed when the ultrasound and the period-based estimate differ by more than a set amount (more than 5 days before 9 weeks 0 days, or more than 7 days from 9 weeks 0 days to 13 weeks 6 days). Once your clinician sets a due date it should not normally be changed again.

If you are 10 weeks 3 days pregnant, you have completed 10 weeks and are in your 11th week. The calculator shows both: completed weeks and days, and the week you are currently in. The first trimester runs to the end of week 13 (13 weeks 6 days), the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days until birth.

Yes. A longer cycle usually means later ovulation, which pushes the due date later; a shorter one moves it earlier. If your cycles vary a lot from month to month, the period-based date is less reliable, and an early ultrasound is the best way to date the pregnancy. Enter your average cycle length, or use the conception date if you know when you ovulated.

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