How to use the due date calculator
- Choose a method. Most people use the first day of their last menstrual period (LMP). If you know the date you conceived, or you had an IVF embryo transfer, choose that instead — it is usually more precise.
- Enter the date. For the LMP method, also enter your usual cycle length so the estimate can be adjusted for longer or shorter cycles.
- Read the results: your due date, how far along you are, your trimester and a table of prenatal milestones.
How due dates are calculated
A typical pregnancy is counted as 40 weeks (280 days) from the first day of the last menstrual period. Adding 280 days to the LMP is known as Naegele's rule, after the German obstetrician Franz Naegele, who described it in the early 1800s. A quick way to do it by hand: add 7 days to the LMP, subtract 3 months and add 1 year.
| Method | Due date |
|---|---|
| Last menstrual period | LMP + 280 days, + (cycle length − 28) days |
| Conception date | Conception + 266 days |
| IVF, day-3 embryo transfer | Transfer + 263 days |
| IVF, day-5 (blastocyst) transfer | Transfer + 261 days |
Naegele's rule assumes a 28-day cycle with ovulation about two weeks after the period starts. If your cycles are longer, you probably ovulated later, so the calculator moves the due date later by the difference — and earlier for shorter cycles. IVF dates are the most exact because the embryo's age on transfer day is known: a day-5 embryo is already five days past fertilization.
Why weeks are counted from your last period
Pregnancy length is measured as gestational age, which starts on the first day of your last period — about two weeks before conception actually happens. That is why you are considered "4 weeks pregnant" around the time you miss your period, and why a conception date or IVF transfer is converted to the matching gestational age before the due date is calculated.
Worked example
If your last period started on January 1, 2026, and your cycle is 28 days, your estimated due date is January 1 + 280 days = October 8, 2026 (the shortcut gives the same answer: January 8, minus 3 months, plus 1 year). With a 32-day cycle, the date moves 4 days later, to October 12, 2026.
Trimesters and term
| Stage | Gestational age |
|---|---|
| First trimester | 0 weeks to 13 weeks 6 days |
| Second trimester | 14 weeks 0 days to 27 weeks 6 days |
| Third trimester | 28 weeks 0 days to birth |
| 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 |
| Post-term | 42 weeks 0 days and beyond |
These are the definitions used by the American College of Obstetricians and Gynecologists (ACOG). The screening windows in the milestone table are typical timings; your provider will schedule tests based on your own care plan.
Only your provider can confirm your due date
This calculator gives an estimate. According to ACOG, an ultrasound measurement of the embryo or fetus in the first trimester is the most accurate way to establish or confirm gestational age, and your provider may change your due date if the ultrasound and your LMP dates disagree. Period dates are especially unreliable if your cycles are irregular, you recently stopped hormonal birth control, or you became pregnant while breastfeeding. Contact your healthcare provider as soon as you think you are pregnant to start prenatal care, and seek care right away for heavy bleeding, severe abdominal pain, or other symptoms that worry you.
Important: results are estimates for general information only and are not medical advice. They cannot replace an assessment by a doctor, registered dietitian or other qualified health professional.
Frequently asked questions
How accurate is my due date?
It is an estimate. Only a small share of babies are born on their exact due date, and a birth anywhere in the full-term window (39 weeks 0 days to 40 weeks 6 days) is considered on time. An early ultrasound gives the most reliable dating.
Why does pregnancy count from my last period instead of conception?
Most people know when their last period started but not exactly when they conceived, so gestational age is measured from the LMP. Conception usually happens about two weeks later, which is why a 40-week pregnancy is about 38 weeks from conception.
My cycles are irregular. Can I still use this?
You can get a rough idea, but the LMP method assumes regular cycles. If your cycles vary a lot, an early ultrasound from your provider will date the pregnancy much more accurately.
Can my due date change?
Yes. Your provider may adjust it after a first-trimester ultrasound if the measurements do not match your period dates. Once a due date has been set from an early ultrasound, it usually is not changed later.
How is an IVF due date calculated?
It is based on the transfer date and the age of the embryo: 263 days after a day-3 transfer or 261 days after a day-5 (blastocyst) transfer. Both are equivalent to 280 days from a matching “last period” date.
Last updated: October 8, 2026