Pregnancy Due Date Calculator
Estimate your due date from the first day of your last period or your conception date — and see how many weeks pregnant you are today and which trimester.
From the first day of your last period
Today's date is taken from your device clock. Estimates only — your midwife or doctor will use the dating scan as the official due date once it's available.
Formula
Two short formulas — pick the tab for what you have:
- From LMP:
due date = LMP + 280 days + (cycle length − 28). The cycle adjustment shifts ovulation, and so the due date, by the same number of days. - From conception:
due date = conception + 266 days.
Trimesters use the standard clinical boundaries: trimester 1 ends at 14 weeks 0 days, trimester 2 at 28 weeks 0 days, and trimester 3 runs from there to the due date.
When this calculator helps
This calculator estimates your due date from the first day of your last menstrual period using Naegele's rule, the same default method ACOG uses. It adds 280 days — 40 weeks — to that first day, which is how a full pregnancy is counted from the start of your last period rather than from conception itself. It is the quickest way to get an estimated due date the moment a home test comes back positive.
Think of it as a planning estimate, not a promise. A rough due date helps you map out when to schedule your first prenatal visit, when maternity leave might start, and how the months ahead line up. But it really is an estimate: only about 4% of babies in the US arrive on their exact estimated due date, so treat the result as the center of a range rather than a fixed appointment with your newborn.
How to read your result
The date shown is your estimated due date (EDD) — the day you would reach 40 weeks counting from your last period. Around it sits the term window: ACOG splits this into early term (37 to 38 weeks), full term (39 to 40 weeks), late term (41 weeks), and post-term (42 weeks and beyond). A birth anywhere across roughly 37 to 42 weeks is considered normal, so don't fixate on the single date.
The result also tells you which trimester you are in. The first trimester runs from your last period through week 13, the second from week 14 through week 27, and the third from week 28 until delivery. These are clinical conventions rather than biological milestones, but they're a handy way to track where you are and which prenatal checks — like the anatomy scan or glucose screening — are coming up next.
A worked example
Say the first day of your last period was 02/01/2026 and you have a typical 28-day cycle. Adding 280 days gives an estimated due date of 11/08/2026. If today were 06/01/2026, you'd be about 17 weeks along — well into the second trimester, with the anatomy scan at 18 to 22 weeks just ahead. Move that start date up by a week and the entire window moves with it, which is why pinning down the right first-day date matters.
Common mistakes to avoid
A handful of easy mix-ups can shift the estimate by days or weeks, so it's worth double-checking these before you lean on the number.
- Assuming a 28-day cycle when yours runs consistently longer or shorter — each day over 28 pushes the due date a day later, so enter your actual cycle length.
- Treating the due date as set in stone; it's the middle of a 37-to-42-week term window, not a guaranteed delivery day.
- Mixing up gestational age (counted from your last period) with fetal age (counted from conception, about two weeks less) — prenatal weeks are always quoted as gestational age.
- Relying on a fuzzy memory of your last period when an early ultrasound would give a much more accurate starting point.
How this fits US prenatal care
In the US your LMP-based estimate is just the opening number. Once you start prenatal care with an OB-GYN or a certified nurse-midwife — ACOG suggests the first visit around 8 to 10 weeks — you'll usually have a dating ultrasound. If an early ultrasound (before 14 weeks) disagrees with your last-period date by more than about 5 to 7 days, ACOG recommends the ultrasound date become your official EDD, since first-trimester ultrasound dating is more accurate than menstrual dating.
So if the due date your provider sets from ultrasound differs from the figure here, go with theirs. This calculator is informational only and is not a substitute for prenatal care — always follow the due date your OB-GYN or midwife confirms, and bring any questions or concerns about your pregnancy directly to them.
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Frequently asked questions
How is the estimated due date (EDD) calculated?▾
The American College of Obstetricians and Gynecologists (ACOG) uses Naegele's rule by default: first day of last menstrual period (LMP) plus 280 days (40 weeks). That assumes a 28-day cycle with ovulation on day 14. ACOG's committee opinion on dating recommends that an early ultrasound (before 14 weeks) supersede the LMP date if the two disagree by more than 5–7 days, since first-trimester ultrasound dating is more accurate than menstrual dating.
What if my cycle isn't 28 days?▾
Adjust the cycle length input — each day longer than 28 shifts the estimated due date one day later, since ovulation (and conception) happens proportionally later. The calculator uses (cycle length − 28) as the adjustment. For very irregular cycles a first-trimester dating ultrasound is more reliable; that's the standard your OB-GYN will use to set the official due date if it's available.
Will my baby actually arrive on the due date?▾
Only about 4% of US babies are born on their exact estimated due date. Most arrive within roughly 37–42 weeks of LMP — what ACOG and the NIH define as 'term'. Specifically: early term (37 0/7 to 38 6/7), full term (39 0/7 to 40 6/7), late term (41 0/7 to 41 6/7), and post-term (42 0/7 and beyond). Induction is commonly discussed at 41 weeks and recommended by 42.
When does each trimester start?▾
Trimester 1 runs from LMP through week 13, trimester 2 from week 14 through week 27, and trimester 3 from week 28 to delivery. The boundaries are clinical conventions, not biological events — they roughly mark the end of organogenesis (T1), the more comfortable middle phase (T2), and the third-trimester growth phase (T3). Major prenatal milestones include the first prenatal visit at 8–10 weeks, the anatomy scan at 18–22 weeks, glucose screening at 24–28 weeks, and Group B Strep at 35–37 weeks.
When should I see my OB-GYN about a pregnancy?▾
ACOG recommends scheduling the first prenatal visit between 8 and 10 weeks. At that visit you'll typically have an exam, lab work (blood type, Rh status, infectious-disease screening), and often a dating ultrasound. Earlier contact is fine — and important if you have specific risk factors or pre-existing conditions — but the formal prenatal care schedule kicks off around weeks 8–10 with monthly visits through week 28, then more frequent visits later.