Pregnancy Due Date Calculator
Work out an estimated due date and see how far along you are, with the dates each trimester begins.
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Finding out you are pregnant brings a lot of questions, and “When is the baby due?” is usually the first. This calculator estimates your due date from the date you know: the start of your last period, the date of conception, or an IVF embryo transfer. It also shows how many weeks pregnant you are today and when each trimester starts.
How to use the due date calculator
- Choose what you are calculating from.
- Enter the date.
- If you are using your last period, enter your average cycle length. 28 days is the default.
The results show your estimated due date, how far along you are today, your current trimester, the days to go, an estimated conception date and the dates the second and third trimesters begin.
How the due date is calculated
The standard method is Naegele’s rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). That assumes a 28-day cycle with ovulation on day 14.
Due date = LMP + 280 days + (cycle length − 28)
Conception ≈ LMP + 14 days + (cycle length − 28)The other methods convert to the same scale:
| Starting point | Due date |
|---|---|
| First day of last period (28-day cycle) | + 280 days |
| Conception date | + 266 days |
| Day-3 embryo transfer (IVF) | + 263 days |
| Day-5 embryo transfer (IVF) | + 261 days |
Pregnancy weeks are counted from the LMP date, so “6 weeks pregnant” means 6 weeks since the last period started, about 4 weeks after conception.
Worked example
Your last period started on 1 March 2026 and your cycle is 28 days.
- Due date = 1 March 2026 + 280 days = Sunday 6 December 2026
- Estimated conception: around 15 March 2026
- Second trimester starts at week 13: 31 May 2026
- Third trimester starts at week 27: 6 September 2026
- On 28 October 2026 you would be 34 weeks and 3 days pregnant, with 39 days to go
If your cycle is usually 32 days, ovulation is about 4 days later, so the estimated due date moves to 10 December 2026.
Key milestones by week
| Week | What usually happens |
|---|---|
| 4–6 | A home pregnancy test turns positive |
| 8–14 | First ultrasound (dating scan) |
| 13 | Second trimester begins |
| 18–22 | Mid-pregnancy anatomy scan |
| 27 | Third trimester begins |
| 37 | Baby is considered early term |
| 40 | Estimated due date |
Exact schedules vary by country and by your care provider.
Tips
- Your scan date wins. If an early ultrasound gives a different date, your care provider will usually go by the scan.
- Irregular cycles make the estimate less certain. If your cycle varies a lot, the date from your period is only a rough guide until your first scan.
- Plan for a range. Babies born anywhere from 37 to 42 weeks are considered within the normal range, so prepare a few weeks early.
- Ask about care early. Contact a doctor or midwife as soon as you know you are pregnant to plan check-ups and supplements such as folic acid.
This calculator gives an estimate for general information. It does not replace advice from your doctor or midwife.
Frequently asked questions
How accurate is a due date?
It is an estimate. Only about 4–5% of babies arrive on their exact due date. Most are born within two weeks either side of it. An early ultrasound scan, usually between 8 and 14 weeks, often gives a more accurate date.
Why is pregnancy counted from the last period and not from conception?
Most people know the date their last period started, but not the exact day they conceived. Counting from the period gives a consistent reference point. It means you are already considered two weeks pregnant at conception.
My cycle is longer than 28 days. Does that change my due date?
Yes. With a longer cycle, ovulation usually happens later, so the due date moves later too. The calculator adds the extra days beyond 28 to the estimate. Enter your usual cycle length to account for it.
How is the due date calculated for IVF?
With IVF the embryo's age is known exactly. A day-5 blastocyst transfer gives a due date 261 days after the transfer, and a day-3 transfer gives 263 days.
When do the trimesters start?
Counting from the first day of the last period, the first trimester runs to the end of week 12, the second from week 13 to the end of week 26, and the third from week 27 until birth.