Estimate your due date from your last period, conception date, IVF transfer date or an ultrasound, plus your current week, trimester and key dates.
This is an estimate, not a diagnosis. Only about 4% to 5% of babies arrive on their exact due date, and your doctor may adjust it based on an ultrasound.
The most common method is Naegele's rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). It assumes a 28-day cycle with ovulation around day 14, so the calculator adjusts it for your own cycle length. A longer cycle means ovulation and conception happened later, which pushes the due date later. A shorter cycle moves it earlier, day for day.
The other methods all work back to the same idea. If you know your conception date, for example from ovulation tracking, the due date is 266 days after it. For IVF, the due date is 261 days after a day-5 (blastocyst) transfer, or 263 days after a day-3 transfer. If an ultrasound measured the baby at a certain gestational age on a certain date, the calculator counts forward to 40 weeks from there. Pregnancy weeks are always counted from the first day of the last period, about two weeks before conception, which is why a woman is "4 weeks pregnant" at about the time of a missed period.
Say your last period started on June 20, 2026, and your cycles are 28 days. Adding 280 days gives a due date of Saturday, March 27, 2027. If today is September 15, 2026, you are 87 days along, which is 12 weeks and 3 days, in the last weeks of the first trimester, with 193 days to go. The second trimester begins on September 26 (14 weeks 0 days), the third on January 2, 2027 (28 weeks), and full term (39 weeks) begins on March 20, 2027.
The other methods give nearly the same date for a comparable pregnancy. A conception date of July 4, 2026 gives March 27, 2027. A day-5 IVF transfer on July 10, 2026 gives March 28, 2027, and a day-3 transfer on the same day gives March 30. An ultrasound on August 1, 2026 that measured 8 weeks and 3 days gives March 10, 2027.
Naegele's rule was built on a 28-day cycle. With the same June 20 start, a 35-day cycle moves the due date 7 days later, to April 3, 2027, and a 24-day cycle moves it 4 days earlier, to March 23. The gestational age is adjusted the same way, so on the due date you are always at 40 weeks and 0 days. If your cycles are very irregular, a conception date or an early ultrasound is a more reliable starting point than the LMP.
Dating from an early ultrasound is generally considered the most precise, because it measures the baby directly. In the first trimester, babies grow at a very consistent rate. Doctors usually keep the LMP due date if the ultrasound agrees within roughly a week, and change it if the difference is larger, so your doctor's date may differ from the one here. IVF dating is also very precise, since the exact age of the embryo is known. LMP dating works well when your cycle is regular and you know the date exactly, and it is the least reliable when cycles vary or the date is uncertain.
The first trimester runs through week 13, the second from week 14 through week 27, and the third from week 28 until birth. The three parts are not perfectly equal. Medical groups also describe a baby born at 37 weeks 0 days to 38 weeks 6 days as early term, 39 weeks 0 days to 40 weeks 6 days as full term, 41 weeks as late term, and 42 weeks or later as post-term. That is why the calculator shows the date full term begins, as well as the due date. Most births happen within a couple of weeks either side of the due date.
How accurate is this due date?
It is a solid estimate, but only about 4% to 5% of babies are born on the exact due date, and 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 baby directly and can be more precise than LMP dating, especially if your cycles are irregular. If your doctor gave you a different date, use theirs.
What if my cycles are not a regular 28 days?
Enter your average cycle length in the cycle length field, and the calculator adjusts the due date. If your cycles are very irregular, conception dating or ultrasound dating is generally more reliable than LMP dating.
What is the difference between LMP and conception dating?
LMP dating counts from the first day of your last period, which is easy to know but assumes a standard cycle. Conception dating counts from the day you conceived, about two weeks later on a typical cycle, and can be more accurate if you tracked ovulation.
Does this work for IVF pregnancies?
Yes. Choose the IVF transfer date method and select a day-3 or day-5 transfer. IVF dating is one of the most precise methods because the embryo's exact age is known.
How does the ultrasound method work?
Enter the date of the scan and the gestational age it reported, in weeks and days. The calculator counts forward from that date to 40 weeks. Ultrasounds done in the first trimester are the most reliable for dating.
What are the three trimesters exactly?
The first trimester is weeks 1 through 13, the second is weeks 14 through 27, and the third is week 28 until birth, usually around week 40.
Can the due date change during pregnancy?
It can, usually only in early pregnancy after an ultrasound gives a more precise measurement. Once the date has been set, it is uncommon for a doctor to change it again unless something significant is found.