Answer: Enter your values and the Pregnancy Due Date Calculator returns the exact result instantly — formula, worked example, and a plain-English explanation are included below the tool.
Estimate your baby's due date
Your due date comes from Naegele's rule: take the first day of your last menstrual period (LMP) and add 280 days, which is 40 weeks. The rule dates back to the early 1800s and it's still how nearly every clinic, app, and hospital chart starts the count. There's a mental-math shortcut, too: last period, minus three months, plus seven days, plus a year.
Notice the count starts before conception. Pregnancy weeks are measured from the last period, not from the day egg met sperm, because the period is the date most people can actually pin down. Conception typically happens about two weeks in, at ovulation. So when the calculator says you're 8 weeks pregnant, the embryo has been developing for roughly six.
The 280-day rule quietly assumes a 28-day cycle with ovulation on day 14. Real cycles vary, and the due date should move with them. If your cycles average 33 days, ovulation likely lands near day 19, so everything shifts about five days later. This calculator handles that for you: it adjusts both the due date and the estimated conception date by the difference between your cycle and 28 days. If your cycles are too irregular to average, the LMP method gets shaky and an early ultrasound becomes the better dating tool.
Suppose your last period started on March 10 and your cycles run the textbook 28 days. Add 280 days and the due date lands on December 15. Estimated conception is around March 24. If instead your cycles average 33 days, the calculator adds five more days: due date December 20, conception around March 29. Small input, real difference, which is why the cycle field is worth filling in honestly rather than leaving at the default.
| Trimester | Weeks | What typically happens |
|---|---|---|
| First | 1 – 13 | Organs form, morning sickness peaks, dating ultrasound around 8 – 12 weeks |
| Second | 14 – 27 | Anatomy scan near 20 weeks, movement felt around 18 – 22 weeks, energy usually returns |
| Third | 28 – birth | Fastest weight gain, visits move to weekly from 36 weeks, full term begins at 39 |
Clinics write gestational age as weeks plus days, so "12w3d" means 12 completed weeks and 3 extra days. The calculator's week number works the same way: it counts full weeks since your last period, which is why it can feel like you "lose" a week compared with counting months. A pregnancy is closer to ten lunar months than nine calendar ones. It's also why the third trimester routinely runs past week 40; the due date sits at the 40-week mark, not at the end of the range of normal.
Treat it as the middle of a window, not an appointment. Only about 4 to 5 percent of babies arrive on the day itself, and roughly 8 in 10 arrive within the two weeks either side. First babies tend to run slightly late. Providers may also adjust the date once, early on: if a first-trimester ultrasound disagrees with the period-based date by more than about 5 to 7 days, the scan wins, because early embryos grow at a very predictable rate. After that the date is locked in. Here's how delivery timing is categorized:
| Category | Timing |
|---|---|
| Preterm | Before 37 weeks |
| Early term | 37w0d – 38w6d |
| Full term | 39w0d – 40w6d |
| Late term | 41w0d – 41w6d |
| Postterm | 42 weeks and beyond |
Want the deeper version, including the ultrasound re-dating thresholds doctors follow and how IVF dating works? Read our guide on how due dates are calculated. This calculator is for planning, not diagnosis; confirm dates and care decisions with your provider.
Usually about two weeks after the first day of your last period, at ovulation. With a 28-day cycle that's around day 14; longer cycles push it later. The calculator estimates your conception date by adding 14 days to your last period plus your cycle adjustment. Sperm can survive up to five days, so the fertile window starts before ovulation itself.
Because the standard 280-day rule assumes ovulation on day 14 of a 28-day cycle. If your cycles run 33 days, you likely ovulate around day 19, so conception happened about five days later than the standard rule assumes and your due date shifts five days later too. The calculator applies this adjustment automatically.
An early ultrasound solves it. Between about 8 and 13 weeks, measuring the embryo's crown-rump length dates the pregnancy to within 5 to 7 days, and your provider will set the official due date from that scan. For IVF pregnancies the transfer date gives an exact answer: day-5 transfer plus 261 days.
40 weeks from the last period is the standard count, but full term is defined as 39 weeks 0 days through 40 weeks 6 days. Babies born at 37 to 38 weeks are early term, 41 weeks is late term, and 42 or more is postterm. Only about 1 in 20 babies arrive on the due date itself.