Pregnancy Due Date Calculator
A due date is an estimate of when your baby will arrive, counted as 40 weeks from the first day of your last menstrual period — which is about 38 weeks after conception. This calculator works it out from whichever date you know best: the first day of your last period (using Naegele’s rule and adjusting for a cycle that is not 28 days), the conception date, or an IVF embryo transfer date. It also shows how many weeks and days pregnant you are on any date, which trimester you are in and the dates of some common milestones. Only a small minority of babies are born on their due date; most arrive between weeks 37 and 42, so think of it as the middle of a window.
How to use the pregnancy due date calculator
- Choose the date you know: last period, conception, or an IVF transfer date.
- Enter it. If you chose last period and your cycle isn’t 28 days, enter your usual cycle length.
- Optionally enter an as-of date to see how far along you will be on that day.
- Remember your clinician may adjust the date after an early ultrasound; their dating takes precedence.
Formula
From last period (Naegele’s rule): due date = first day of last period + 280 days (40 weeks), adjusted by (cycle length − 28) days for longer or shorter cycles. From conception: due date = conception date + 266 days. IVF: transfer date + 261 days (day-5 embryo) or + 263 days (day-3 embryo). Gestational age counts from the first day of the last period, so it runs about two weeks ahead of the actual age of the embryo.
Worked examples
From the last period
If your last period began on 1 January 2026 and your cycle is 28 days, your due date is Thursday, October 8, 2026. On 12 March 2026 you would be 10 weeks 0 days pregnant.
A longer cycle
With a 32-day cycle the same last period gives Monday, October 12, 2026 — four days later, because ovulation (and conception) happened later in the cycle.
From conception or IVF
Conception on 15 January 2026 gives Thursday, October 8, 2026, and a day-5 IVF transfer on 20 January 2026 gives Thursday, October 8, 2026.
Pregnancy milestones and what a due date means
Trimesters
| Stage | Weeks |
|---|---|
| First trimester | Weeks 1–13 |
| Second trimester | Weeks 14–27 |
| Third trimester | Week 28 to birth |
| Early term / full term | Weeks 37–42 (term) |
Why only some babies arrive on the day
Even with a perfect estimate, labour starts when the baby and body are ready, which varies from person to person. A first pregnancy often runs slightly longer than the due date. If you pass 41–42 weeks, your care team will discuss monitoring and induction.
See also the ovulation calculator and conception calculator.
Practice problems
Try these yourself first, then check your answer. The answers come from the calculator above.
- The first day of the last period was 14 April 2026 and the cycle is 30 days. What is the due date?
Show answer
Estimated due date: Thursday, January 21, 2027 · Gestational age on the “as of” date: 6 weeks 6 days
Common mistakes to avoid
- Entering the conception date when the tool is set to “last period”. Choose the method that matches the date you have.
- Treating the due date as a deadline. Only about 4–5% of babies are born on it; the normal range is 37–42 weeks.
- Ignoring cycle length. If your cycle is much longer or shorter than 28 days, last-period dating can be off by a week or more.
- Overriding your clinician. An early ultrasound measurement is often more accurate and is used for official dating.
- Using this as medical advice. It is an estimate for planning.
Frequently asked questions
How is a due date calculated?
By adding 280 days (40 weeks) to the first day of your last menstrual period — Naegele’s rule — or by adding 266 days to the conception date.
How accurate is a due date?
It is an estimate. Around 4–5% of babies are born on the date; most arrive between 37 and 42 weeks.
Why does pregnancy start before conception?
Because the first day of the last period is the one date most people know. Conception usually happens about two weeks later, so pregnancy is dated from the period.
Does cycle length change my due date?
Yes. If you ovulate later in a longer cycle, conception and the due date are later too; the calculator adjusts by the difference from 28 days.
What if my ultrasound gives a different date?
Your midwife or doctor may re-date the pregnancy from an early scan. Follow their date.
Is this medical advice?
No. It is an educational estimate; consult your healthcare provider for prenatal care.
Sources and further reading
Reviewed September 26, 2026 by the SolveCalcPro editorial team. Found a mistake? Tell us; see our editorial policy.