Due date calculator
Enter one date and see your estimated due date, how far along you are today, and your full-term window.
Updated October 13, 2026 · sources checked October 13, 2026
In short
Your estimated due date is 280 days (40 weeks) after the first day of your last period, adjusted if your cycle is longer or shorter than 28 days. After IVF it is 261 days after a day-5 embryo transfer, or 263 days after a day-3 transfer. A first-trimester scan is the most accurate way to confirm it.
How the calculation works
By convention, obstetricians count pregnancy from the first day of your last menstrual period, not from conception. The estimated due date is 280 days after that day. ACOG explains that this convention assumes a regular 28-day cycle with ovulation on day 14, which is why the calculator shifts the date when you tell it your cycle is longer or shorter.
| You know | The calculator does |
|---|---|
| The first day of your last period | Adds 280 days, then adjusts for your cycle length |
| The date of conception | Adds 266 days (280 minus the 14 days before ovulation) |
| An IVF transfer date | Adds 261 days for a day-5 embryo, 263 days for a day-3 embryo (ACOG) |
| A scan date and the age given at the scan | Counts back to day 0, then adds 280 days |
Which date to trust
If your scan and your period give different dates, follow the date your midwife or doctor gives you. ACOG recommends changing the due date when a scan done before 14 weeks differs from the period-based date by more than 7 days, and by smaller margins for very early scans. In the UK, the NHS offers a dating scan between 11 and 14 weeks.
Weeks, trimesters and the term window
The first trimester runs to 13 weeks and 6 days, the second to 27 weeks and 6 days, and the third from 28 weeks to birth. Few babies arrive exactly on their due date: think of it as the middle of a window, not an appointment.
Is it normal?
Your provider may move your due date once or twice in early pregnancy. That is a normal part of dating, not a sign that something is wrong.
Next steps
- See your pregnancy week by week calendar with every week's dates.
- Check how many weeks pregnant you are today.
Questions parents ask
How accurate is a due date?
It is an estimate. ACOG notes that a first-trimester ultrasound, measured up to 13 weeks and 6 days, is the most accurate way to establish or confirm how far along you are, so your provider may adjust the date after that scan.
My cycle is 32 days. Does that change my due date?
Yes. The 280-day rule assumes ovulation on day 14 of a 28-day cycle. The calculator moves the date by the difference, so a 32-day cycle gives a due date about 4 days later.
What is full term?
ACOG calls 37 weeks to 38 weeks and 6 days early term, 39 weeks to 40 weeks and 6 days full term, 41 weeks to 41 weeks and 6 days late term, and 42 weeks or more post-term.
Sources
- ACOG Committee Opinion No. 700, Methods for Estimating the Due Date (2017) checked October 13, 2026
- ACOG Committee Opinion No. 579, Definition of Term Pregnancy checked October 13, 2026
- NHS — Your antenatal appointments checked October 13, 2026
Mamavela explains; it does not replace your midwife, doctor or pediatrician, who know your situation. If something worries you, call them. In an emergency, call 911 (US), 999 (UK) or 112.