Due date calculator.
Enter the first day of your last period, a conception date or an IVF transfer date. You get an estimated due date, how far along you are today and the dates each trimester begins.
This tool is educational and does not replace your midwife or doctor. Nothing you enter leaves your device.
How this due date calculator works
Pregnancy is dated from the first day of your last menstrual period (LMP), about two weeks before conception. The standard rule adds 280 days to that date. If your cycle is not 28 days, ovulation probably happened earlier or later, so the calculator moves the date by the difference.
| Starting point | What is added |
|---|---|
| First day of last period | 280 days, plus or minus (cycle length − 28) |
| Conception date | 266 days |
| IVF day-5 transfer | 261 days |
| IVF day-3 transfer | 263 days |
What the dates mean
The trimesters here follow the common convention used by ACOG: the first trimester runs to 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days. “Full term” starts at 39 weeks 0 days.
A due date is the middle of a window, not a deadline. The NHS describes a normal pregnancy as lasting 37 to 42 weeks, and your midwife or doctor may adjust the date after your dating scan. If they do, use theirs.
What to do next
- See how many weeks pregnant you are on a timeline.
- Read which skincare ingredients are usually avoided in pregnancy.
- Start the hospital bag checklist. The NHS suggests having it packed at least 3 weeks before your due date.
Questions, answered
How is a due date calculated?
The standard method adds 280 days (40 weeks) to the first day of your last menstrual period, assuming a 28-day cycle. This calculator shifts the date by the difference if your cycle is longer or shorter. From a known conception date it adds 266 days.
How accurate is a due date?
It is an estimate. The NHS says pregnancy normally lasts from 37 to 42 weeks from the first day of your last period, and only about 1 in 20 babies arrive on their due date. A first-trimester ultrasound can estimate the date more accurately, and your midwife or doctor may change it after your scan.
How do I calculate a due date after IVF?
Use the transfer date. For a day-5 embryo (blastocyst) the due date is 261 days after transfer; for a day-3 embryo it is 263 days after transfer. Your clinic's date takes priority.
When is a baby full term?
ACOG defines early term as 37 weeks 0 days to 38 weeks 6 days, full term as 39 weeks 0 days to 40 weeks 6 days, late term as 41 weeks 0 days to 41 weeks 6 days, and post-term as 42 weeks and beyond.
What if I do not know the date of my last period?
The NHS advises speaking to a midwife or GP. A dating scan will estimate how many weeks pregnant you are.
Sources
- ACOG Committee Opinion 700: Methods for Estimating the Due Date
- NHS: Due date calculator
- ACOG Committee Opinion 579: Definition of Term Pregnancy
- NHS: Pack your bag for labour (1 in 20 babies arrive on their due date)
Sources last checked on 11 November 2026.