How this pregnancy due date calculator works
A pregnancy is dated from the first day of the last menstrual period (LMP), not from conception. By convention the estimated due date is 280 days, or 40 weeks, after that day. Every method in this due date calculator works back to that same starting point and then adds 280 days.
| Method | Estimated due date |
|---|---|
| Last period | First day of last period + 280 days + (cycle length − 28) |
| Ultrasound | Ultrasound date − gestational age at the scan + 280 days |
| Conception date | Conception date + 266 days |
| IVF transfer date | Transfer date + 266 days − embryo age (3, 5 or 6 days) |
The 280-day rule assumes a 28-day cycle with ovulation on day 14. If your cycles are usually longer or shorter, ovulation shifts by about the same number of days, so the calculator moves the due date by the difference between your cycle length and 28. A 35-day cycle moves it 7 days later; a 24-day cycle moves it 4 days earlier.
How many weeks pregnant am I?
Gestational age is the time since the start of the last period, written as completed weeks plus days. Someone who is 12 weeks 3 days along has finished 12 full weeks and is in week 13. The calculator shows both, along with the same span in calendar months.
Trimesters follow the pregnancy week: weeks 1 to 12 are the first trimester, weeks 13 to 27 the second, and week 28 onward the third. Progress is the share of the 280 days that have passed. Baby length and weight are commonly published averages for each week and vary widely from one pregnancy to the next.
The likely conception date is 14 days after the start of a standard cycle, which is 266 days before the due date. It is shown for the last period and ultrasound methods.
Term stages near the due date
A due date is the middle of a window, not a deadline. Births from 37 weeks on are grouped into four stages:
- Early term: 37 weeks 0 days to 38 weeks 6 days
- Full term: 39 weeks 0 days to 40 weeks 6 days
- Late term: 41 weeks 0 days to 41 weeks 6 days
- Postterm: 42 weeks 0 days and beyond
Frequently asked questions
How accurate is a due date?
It is an estimate. Only a small share of babies arrive on the exact date, and most are born within about two weeks on either side of it.
Which method should I use to calculate my due date?
Use the last period method when you know the date and your cycles are regular. If you have had a dating ultrasound, enter the scan date and the gestational age measured that day; a first-trimester ultrasound is the most accurate way to date a pregnancy. Use the IVF transfer date after an embryo transfer, since the embryo's age is known exactly.
How does an IVF due date calculator differ?
With IVF there is no need to guess when ovulation happened. The due date is 266 days after fertilization, so the calculator subtracts the embryo's age from that: a day 5 transfer is due 261 days after the transfer, and a day 3 transfer 263 days after it.
Can my due date change?
Yes. A provider may revise it after an early ultrasound when the measured gestational age differs from the date based on your last period.
Limitations
This calculator gives general estimates for a single pregnancy and is not medical advice. Twins and other multiples are usually delivered earlier. Your obstetric provider's dating takes priority over any estimate here.
What's changed
- Launched.
References
- ACOG Committee Opinion No. 700, Methods for Estimating the Due Date
280 days from the last period, ultrasound dating, and dating by embryo age and transfer date
- ACOG Committee Opinion No. 579, Definition of Term Pregnancy
early term, full term, late term and postterm ranges