How to use the due date calculator
- Choose what to base the estimate on: the first day of your last menstrual period (LMP), your conception date or your IVF transfer date.
- Enter the date. For LMP, add your average cycle length; for IVF, choose a 3-day or 5-day embryo transfer.
- See your estimated due date, how many weeks pregnant you are, your trimester and a timeline of key milestones.
How your due date is calculated
Most healthcare providers use Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle with ovulation on day 14.
- From conception: due date = conception date + 266 days (38 weeks).
- From a 3-day embryo transfer: due date = transfer date + 263 days.
- From a 5-day embryo transfer: due date = transfer date + 261 days.
Example: if your last period started on January 1, 2026 and your cycle is 28 days, your estimated due date is October 8, 2026.
Why pregnancy is counted from your last period
Pregnancy is measured in gestational age, which starts on the first day of your last period — about two weeks before conception actually happens. That's why at the moment of conception you are already considered "two weeks pregnant", and why a full-term pregnancy is described as 40 weeks.
Trimesters of pregnancy
| Trimester | Weeks | What's happening |
|---|---|---|
| First | 1 – 13 | Major organs begin to form; common symptoms include nausea and fatigue. |
| Second | 14 – 27 | Many people feel more energetic; you may feel the first movements and have an anatomy scan. |
| Third | 28 – 40+ | The baby gains weight rapidly and prepares for birth. |
How accurate is my due date?
A due date is an estimate. Only a small share of babies are born on the exact day, and a pregnancy is considered full term from 39 weeks 0 days to 40 weeks 6 days. An early ultrasound, especially in the first trimester, is the most accurate way to date a pregnancy, and your provider may adjust your due date based on it.
Frequently asked questions
How is a due date calculated?
The standard method, Naegele's rule, adds 280 days (40 weeks) to the first day of your last menstrual period. If your cycle is longer or shorter than 28 days, the date is adjusted by the difference.
Can I calculate my due date from the conception date?
Yes. Add 266 days (38 weeks) to the date of conception. If you are not sure of the exact date, using the first day of your last period is usually more reliable.
How many weeks is a full-term pregnancy?
A full-term pregnancy lasts from 39 weeks 0 days to 40 weeks 6 days of gestational age. Babies born from 37 weeks are considered early term, and from 41 weeks late term.
How accurate are due date calculators?
They give a good estimate, but only about 4% of babies are born on their due date. An early ultrasound gives the most accurate dating, and your doctor or midwife may update your due date based on it.
How do I calculate my due date after IVF?
Add 263 days to a 3-day embryo transfer date or 261 days to a 5-day (blastocyst) transfer date. Select "IVF transfer date" above to calculate it automatically.
This calculator provides general information only and is not a substitute for professional medical advice, diagnosis or treatment. Always consult a doctor or other qualified health provider about your individual situation.
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