Estimate your due date from your last period, conception date, or IVF transfer date - plus your current week and trimester.
This gives an estimate, not a diagnosis. Only about 5% of babies arrive on their exact due date - your doctor may adjust this based on an ultrasound.
The most common method, called Naegele's Rule, adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). It's adjusted here for cycle length, since a longer or shorter cycle shifts when ovulation - and therefore conception - actually happened relative to a "standard" 28-day cycle.
If you know your conception date more precisely (for example, from ovulation tracking), the due date is 266 days after conception. For IVF pregnancies, the due date is calculated from the transfer date and the embryo's age at transfer - 261 days after a day-5 (blastocyst) transfer, or 263 days after a day-3 transfer.
Pregnancy is typically divided into three trimesters: the first runs through week 13, the second from week 14 through week 27, and the third from week 28 until birth, usually around week 40. These aren't exactly three equal parts - the last trimester runs slightly longer than the other two.
Naegele's Rule was originally built around a 28-day cycle with ovulation on day 14. If your cycles run longer, ovulation - and conception - happens later, which pushes your due date later too; shorter cycles shift it earlier. This calculator adds or subtracts the difference from 28 days automatically.
Say your last period started June 20, with a typical 28-day cycle. Naegele's Rule puts your due date at March 27 of the following year. If today is September 15, that's about 12 weeks and 3 days along - the tail end of the first trimester, with roughly 193 days left to go. For comparison, an IVF pregnancy with a day-5 transfer on July 10 lands on almost the same due date, March 28 - which makes sense, since a day-5 transfer is about 19 days past a theoretical last period.
How accurate is this due date?
It's a solid estimate, but only a small share of babies are actually born on their exact due date - most arrive within about two weeks before or after. Think of it as a target date, not a deadline.
Why does my doctor's due date differ from this calculator?
Doctors often adjust the due date based on a first-trimester ultrasound, which measures the fetus directly and can be more precise than LMP dating alone, especially if your cycles are irregular. If your doctor gave you a different date, trust theirs.
What if my cycles aren't a regular 28 days?
Enter your actual average cycle length in the cycle length field - the calculator adjusts the due date to account for it. If your cycles are very irregular, conception or ultrasound dating will generally be more reliable than LMP dating.
What's the difference between LMP and conception dating?
LMP dating counts from the first day of your last period, which is easier to know precisely but assumes a standard cycle. Conception dating counts from the estimated day you actually conceived, about two weeks later on a typical cycle, which can be more accurate if you tracked ovulation.
Does this work for IVF pregnancies?
Yes - select the IVF transfer date method and choose whether it was a day-3 or day-5 transfer. IVF dating tends to be one of the most precise methods, since the embryo's exact age is known at transfer.
What are the three trimesters exactly?
First trimester: weeks 1 through 13. Second trimester: weeks 14 through 27. Third trimester: week 28 until birth, typically around week 40.
Can the due date change during pregnancy?
It can, usually only in early pregnancy after an ultrasound provides a more precise measurement. Once that early estimate is set, it's uncommon for a doctor to move it again unless something significant is found later on.