How to calculate your due date from your last period
Your estimated due date is 280 days (40 weeks) after the first day of your last menstrual period. If your last period started on 1 March 2026, your due date is Sunday 6 December 2026, and on 6 October 2026 you are 31 weeks and 2 days pregnant. Enter your own date in the due date calculator.
Why count from your last period?
Most people don't know the exact day they conceived, but they do know when their last period began. Doctors and midwives therefore date a pregnancy from the first day of the last menstrual period (LMP). In a regular 28-day cycle, ovulation and conception happen about two weeks later, so at conception you are already counted as "two weeks pregnant".
A pregnancy lasts on average 280 days, or 40 weeks, from the LMP, which is 266 days from conception.
Naegele's rule
The classic shortcut, used for well over a century: take the first day of your last period, add one year, subtract three months and add seven days.
For 1 March 2026: + 1 year = 1 March 2027, − 3 months = 1 December 2026, + 7 days = 8 December 2026. Counting exactly 280 days gives 6 December 2026. Because months have different lengths, Naegele's rule can differ by a day or two; the calculator counts exactly 280 days.
Adjusting for cycle length
The 280-day rule assumes a 28-day cycle with ovulation on day 14. With a longer cycle, ovulation comes later, so the due date moves later by the same number of days. With a 32-day cycle, add 4 days: 10 December 2026 instead of 6 December. With a 25-day cycle, subtract 3 days.
How many weeks pregnant am I?
Count the days from the first day of your last period to today and divide by 7. From 1 March to 6 October 2026 is 219 days: 219 ÷ 7 = 31 weeks and 2 days, often written as 31+2. You are then in your 32nd week of pregnancy, just as someone who is 31 years old is in their 32nd year.
The dating scan
An early ultrasound, usually between 8 and 14 weeks, measures the baby and gives a more accurate estimate than the period date. If the two differ by more than a few days, the scan date is normally used. Your due date may therefore change after your first scan.
Key milestones
| Week | Milestone |
|---|---|
| 8–14 | dating scan |
| 12 | end of first trimester |
| 18–21 | anatomy scan |
| 24 | viability milestone |
| 27 | end of second trimester |
| 37–42 | full term; labour can start any time |
Only about 4 to 5% of babies arrive on their due date. Most are born between 38 and 42 weeks, so think of it as a due window rather than a single day.
IVF and known conception dates
If you know your conception date, add 266 days. For IVF, your clinic will calculate the due date from the embryo transfer date and the age of the embryo: for a day-5 transfer, add 261 days to the transfer date.
Frequently asked questions
Is the due date 40 weeks from conception?
No. It is 40 weeks from the first day of your last period, which is about 38 weeks from conception.
How accurate is a due date?
It is an estimate. Most babies are born within two weeks before or after it, and first babies tend to arrive slightly later on average.
What if I don't know the date of my last period?
Then an ultrasound scan is used to date the pregnancy. Measurements in the first trimester are the most accurate.
When does the second trimester start?
After week 12, so from 12 weeks and 0 days. The third trimester starts after week 27.