How the due date calculator works
A due date is an estimate of when a full-term pregnancy will reach about 40 weeks. This tool gives you three ways to reach that estimate, depending on what you know: the first day of your last period, the date you conceived, or the date of an IVF embryo transfer. Whichever you pick, the calculator also shows your current gestational age, the trimester you are in, your estimated conception date, and how many days remain until the due date.
- From LMP: due date = LMP + 280 days + (cycle length − 28 days)
- From conception: due date = conception date + 266 days
- From IVF (Day-5 transfer): due date = transfer date + 261 days
Naegele’s rule: LMP plus 280 days
The most common method starts from the first day of your last menstrual period (LMP), not from the day you conceived. A full-term pregnancy is counted as 40 weeks, or 280 days, from that LMP date. This is Naegele’s rule: add a year to the LMP, subtract three months, and add seven days, which comes out to about 280 days.
For example, a last period starting on 5 January 2026 with a standard 28-day cycle gives an estimated due date of Monday, October 12, 2026, exactly 280 days later.
Adjusting for cycle length
Naegele’s rule quietly assumes a 28-day cycle with ovulation around day 14. Real cycles vary, so the calculator adjusts the LMP estimate by the difference between your cycle and 28 days. If your cycle runs longer than 28 days you probably ovulate later, so the due date moves later by the same number of days; a shorter cycle pulls it earlier.
Take the same 5 January 2026 last period but a 30-day cycle. That adds 2 days, moving the estimated due date to Wednesday, October 14, 2026 instead of October 12. Cycle length only affects LMP dating, since the conception and IVF methods already fix the timing of ovulation or transfer.
Conception and IVF dating
If you know when you conceived, dating is more direct. Actual pregnancy from conception lasts about 266 days (38 weeks), which is roughly the 280-day LMP figure minus the two weeks before ovulation. So the calculator adds 266 days to a conception date.
IVF is the most precise case because the transfer date is known exactly. For a Day-5 blastocyst transfer, the calculator adds 261 days (266 days from conception, minus the 5 days the embryo already developed in the lab). A Day-3 transfer would add 263 days instead. The tool defaults to the more common Day-5 transfer.
Gestational age and trimesters
Gestational age is how far along the pregnancy is, measured in weeks and days counting from the LMP (or the LMP-equivalent, about 14 days before conception), not from conception itself. Because it counts from LMP, gestational age runs roughly two weeks ahead of the embryo’s actual age. Pregnancy is commonly split into three trimesters by gestational week:
- First trimester: weeks 0 to 13.
- Second trimester: weeks 14 to 27.
- Third trimester: weeks 28 and beyond, up to birth.
The calculator shows your current gestational age (for example, “12 weeks, 3 days”) and the trimester it falls in, based on today’s date compared with your estimated LMP.
Accuracy and a note on medical advice
Estimated due dates are exactly that: estimates. Only about 4% of babies arrive on the predicted day, and most healthy births happen in the weeks around it. LMP dating assumes you recall the date correctly and ovulate on a regular schedule, so irregular cycles or an uncertain last period can shift the true date. An early ultrasound is generally the most reliable way to confirm dating, and your provider may adjust the date accordingly.
Please use this calculator for general information and planning only. It is an estimate, not a diagnosis, and not a substitute for professional care. For anything that affects your health or your pregnancy, talk with a qualified healthcare provider who can review your full history and confirm your dates.