Clear Pregnancy Due Date Calculator
Three ways to estimate a due date — from the last period, the conception date, or an ultrasound measurement — plus your current week, days remaining and the trimester milestones along the way.
How it works
A full-term pregnancy is counted as 280 days (40 weeks) from the first day of the last menstrual period — Naegele's rule — even though conception typically happens about 14 days later, which is why the conception method uses 266 days. The ultrasound method works backwards from the gestational age measured at the scan: due date = scan date + (280 − age in days). Gestational age "today" is simply the number of days since the (real or implied) LMP, split into weeks and days.
When to use this
- You have just had a positive test and want an estimated due date before your first appointment is even booked.
- You know the conception date — from tracking or IVF — and the last-period method does not apply cleanly.
- A scan gave you a gestational age and you want the due date and milestones that follow from it.
- You are planning leave, travel or childcare and need the trimester dates, not just one day at the end.
How to use it, step by step
- Pick the mode that matches what you actually know: last period, conception date, or ultrasound.
- Enter the date — for ultrasound, also the gestational age in weeks and days as written on the scan.
- Read the estimated due date at the top; it is an estimate, and only a small share of babies arrive on it.
- Use the milestone rows for the end of each trimester and the 37-week full-term date.
- Let your clinician's dating, especially an early ultrasound, override anything a calculator says.
What the inputs mean
| Input | What it is | Typical values |
|---|---|---|
| Last menstrual period | First day of the last period; the starting point for Naegele's rule | The date most people use first |
| Conception date | Ovulation or conception day, if known | Due date is counted as 266 days from here |
| Ultrasound date and gestational age | When the scan happened and the age it measured, in weeks plus days | Earliest scans, before 12 weeks, date a pregnancy most accurately |
Another worked example
With a last period starting Monday, March 2, 2026, Naegele's rule puts the estimated due date on Monday, December 7, 2026. The milestones the tool lists along the way: the first trimester ends Sunday, June 7, 2026, the second trimester ends Sunday, September 13, 2026, and the 37-week full-term mark falls on Monday, November 16, 2026. All of it is an estimate from a population rule — a dating scan can and does move these dates.
Frequently asked questions
How accurate is a due date from my last period?
Naegele's rule assumes a regular 28-day cycle with ovulation on day 14. If your cycles are longer, shorter or irregular, the estimate can be off by a week or more — which is why clinics confirm dating with an early ultrasound.
Why do the three methods give different dates?
They measure from different starting points: the last period (day 0 of the count), conception (about day 14), or a size measurement at a scan. Small differences of a few days are normal and not a cause for concern.
What week of pregnancy am I in?
Count the days since the first day of your last period and divide by 7. For example, 63 days is exactly 9 weeks. The LMP mode above shows your current week and day automatically.
When does each trimester end?
The first trimester ends at 13 weeks 6 days, the second at 27 weeks 6 days, and the third runs from 28 weeks until delivery. The table above gives the calendar dates for your pregnancy.
🌸 Planning ahead?
The Ovulation Calculator maps your fertile window and the next three cycles.
What does the 37-week full-term row actually mean?
It is the date you reach 37 weeks, the point from which a birth is generally classed as full term rather than preterm. It is the start of the normal delivery window, not a second due date — babies arrive across the following weeks, and the estimated due date itself sits at 40 weeks.
What if I do not know my last period date?
Use one of the other modes. If you tracked ovulation or conceived through IVF, the conception mode counts 266 days from that date. If you have had a scan, the ultrasound mode works backwards from the age it measured. If you have none of the three, a clinician's dating scan is the reliable route — guessing an LMP date just to fill the box produces a confidently wrong answer.