How to use the due date calculator
- Choose what to count from: your last period, a due date you already have, a known conception date, an IVF transfer, or an ultrasound scan.
- Enter the date. For a last period, also choose your usual cycle length; for IVF, the embryo age; for a scan, the age it gave in weeks and days.
- Press Calculate for your due date, which week of pregnancy you are in, how far along you are, a timeline, and a week-by-week calendar with the dates of each week and its milestones.
How a due date is calculated
Pregnancy is counted from the first day of the last menstrual period (LMP), about two weeks before conception. A typical pregnancy lasts 280 days – 40 weeks – from that day. This is Naegele's rule:
\text{due date} = \text{LMP} + 280 \text{ days} + (\text{cycle length} - 28)The cycle adjustment moves the date later for longer cycles, because ovulation comes later. The other methods work back to the same starting point:
| Based on | Due date |
|---|---|
| Last period | LMP + 280 days, adjusted for cycle length |
| Conception date | conception + 266 days (38 weeks) |
| IVF, day-3 embryo | transfer date + 263 days |
| IVF, day-5 embryo | transfer date + 261 days |
| IVF, day-6 embryo | transfer date + 260 days |
| Ultrasound | scan date − age at scan + 280 days |
| Known due date | works back 280 days to build the week-by-week calendar |
January 10, 2026 + 280 days = October 17, 2026, plus 2 days for the longer cycle = Monday, October 19, 2026.
Weeks of pregnancy
"Week 23" means you are 22 weeks and some days pregnant – the same way your first year of life is "age 0". Week 1 starts on the first day of your last period, so conception falls in week 3 and the due date at the end of week 40.
Which method is most accurate?
- IVF transfer dates are the most precise, because the embryo's age is known exactly.
- An early ultrasound (before 14 weeks) measuring the baby's crown-rump length is the standard way to confirm or correct a due date. Doctors usually change an LMP date if the scan differs by more than about 5–7 days in the first trimester.
- Last period works well with regular cycles and a clearly remembered date.
- Conception date is often uncertain, because sperm can live up to five days before ovulation.
Trimesters and term
| Stage | Weeks |
|---|---|
| First trimester | 0 to 13 weeks 6 days |
| Second trimester | 14 weeks to 27 weeks 6 days |
| Third trimester | 28 weeks to birth |
| Early term | 37 weeks to 38 weeks 6 days |
| Full term | 39 weeks to 40 weeks 6 days |
| Late term | 41 weeks to 41 weeks 6 days |
| Post-term | 42 weeks and later |
Term definitions follow the American College of Obstetricians and Gynecologists. Only about 1 in 20 babies is born on the due date itself, so it helps to think of a due window from 37 to 42 weeks.
Counting down
To count the days to your due date or between two scans, use the days until calculator or the weeks between dates calculator. If you are still trying to conceive, the ovulation calculator shows your fertile days.
This calculator gives an estimate for planning. Your midwife or doctor will confirm your due date, usually at the first scan.
Frequently asked questions
How is my due date calculated?
By adding 280 days (40 weeks) to the first day of your last period, adjusted for your cycle length. From a conception date, add 266 days.
How many weeks pregnant am I?
Count the weeks since the first day of your last period. The result shows it for today, for example "14 weeks 2 days".
Why does pregnancy count from my last period?
The date of conception is rarely known, while the start of a period usually is. So pregnancy is counted from it, and the first two weeks happen before conception.
How accurate is a due date?
It is an estimate: only about 5% of babies arrive on it. Most are born between 37 and 42 weeks. An early ultrasound makes the estimate more precise.
Can my due date change?
Yes. If an early ultrasound measures your baby as more than about a week different from your period dates, your doctor or midwife may move it.