Due Date Calculator
Work out your estimated due date from your last period, a known conception date or an early ultrasound, and see how many weeks and days pregnant you are today. Every method uses the same 40-week clock your clinician uses.
Enter your details above — results update as you type.
Formula and calculation
Worked example
With a last period that started on April 10, 2026 and a 28-day cycle, the due date is Friday, January 15, 2027 — 280 days later. On October 16, 2026 that makes you 27 weeks 0 days pregnant: Second trimester, 91 days to go, and 67.5% of the 40 weeks behind you.
Why the date matters
Dating is not bookkeeping — it decides when tests and checks are scheduled. Screening windows, the anatomy scan, growth checks and any discussion about induction are all tied to gestational age, and being a week out can shift a decision. That is why an early scan matters: a first-trimester measurement is accurate to about five days, while a later scan can be off by two weeks or more. If you are given a new date after a scan, the new one wins — enter the scan details here and compare.
What the results mean
- Estimated due date
- Your estimated due date (EDD) — the day that marks 40 weeks of pregnancy. Fewer than 1 in 20 babies arrive on it, so treat it as the centre of a window rather than an appointment.
- You are
- How far along you are today, written the way clinicians write it: weeks plus days, counted from the first day of your last period. A 12-week pregnancy is about 10 weeks after conception.
- Trimester
- Which trimester you are in: the first runs to the end of week 13, the second covers weeks 14 to 27, and the third starts at week 28.
- Days to go
- Days from today to the estimated due date. A negative number means the due date has passed — going past 40 weeks is common and is monitored by your clinician.
- Pregnancy progress
- How much of the 40-week clock has run: 100% is the due date. Past 100% does not mean something is wrong, only that the estimate has been passed.
Frequently asked questions
How is the due date calculated?
By convention, pregnancy is counted 280 days (40 weeks) from the first day of your last period — Naegele's rule. If the conception date is known, the same date is reached by adding 266 days, because ovulation is assumed to happen about 14 days into the cycle. Both are written into ACOG's guidance on estimating the due date.
How accurate is the due date?
It is an estimate, not a deadline — most births happen either side of it, and anything from 37 to 42 weeks is considered term. The dating methods differ in precision: a known conception date is exact, a first-trimester ultrasound is usually within a few days, and last-period dating is least accurate because cycles and ovulation vary.
What if my cycles are longer or shorter than 28 days?
The due date shifts by the difference. A 28-day cycle is the baseline; a 35-day cycle moves the estimate about a week later (ovulation happens later), and a 24-day cycle moves it a few days earlier. Enter your usual cycle length in the last-period tab and the calculator adjusts the date for you.
What does “weeks and days” pregnant mean?
Clinicians count gestational age from the first day of your last period, not from conception, because the exact conception date is usually unknown. So a 12-week pregnancy is roughly 10 weeks after conception, and a baby born at 40 weeks has developed for about 38 weeks.
What if I don't know my last period date?
Use the ultrasound tab instead: enter the scan date and the gestational age written on the report, and the calculator converts it to a due date. That is the standard fallback — a dating scan before 14 weeks is more reliable than a half-remembered period date, and studies show many people recall their last period inaccurately.