Pregnancy Due Date Calculator
Enter the first day of your last period, your conception date, or a due date you already know to estimate your due date, current week of pregnancy, and key milestones.
Your estimated due date will appear here
Choose a method and enter a date, then click calculate.
Quick Answer
A pregnancy due date calculator estimates when your baby is due, based on a full-term pregnancy of about 280 days from your last period, or 266 days from conception. It also shows your current week of pregnancy, trimester, and key milestone dates.
How It Works: Formula & Variables
Due Date = Last Period + 280 days (or Conception + 266 days)
- Last Period Method
- Adds 280 days (40 weeks) to the first day of your last period, adjusted for your average cycle length.
- Conception Method
- Adds 266 days (38 weeks) to your estimated or known conception date.
- Current Week
- Counted from today's date back to the start of your pregnancy, based on whichever method you used.
Both methods land on the same estimated due date for a textbook 28-day cycle, since ovulation typically happens around day 14. The gap between them accounts for the roughly two weeks between your period starting and conception actually occurring.
Worked Examples
Example 1: Regular 28-day cycle
If your last period started on January 1st and you have a typical 28-day cycle, adding 280 days puts your due date around October 8th. Your estimated conception date would fall around January 15th, roughly two weeks after your period began.
Example 2: Longer cycle
If your cycles run 35 days instead of 28, ovulation happens later, so your due date shifts back by roughly the same 7-day difference. Entering your actual cycle length instead of leaving the default keeps the estimate closer to your real ovulation timing.
Key Concepts
Due dates are estimates, not deadlines: A full-term pregnancy can range anywhere from 37 to 42 weeks, so the calculated date is a midpoint estimate rather than a fixed appointment.
Ultrasound dating often overrides period-based dating: If your doctor's ultrasound estimate differs from your last-period estimate by more than about a week, the ultrasound date is usually the one that sticks for the rest of the pregnancy.
Trimesters mark different stages: The first trimester covers early development and the highest miscarriage risk, the second is often called the most comfortable stretch, and the third covers rapid growth as your body prepares for birth.
Common Mistakes
Treating the due date as exact: Planning travel, work leave, or hospital logistics around the precise calculated day, rather than the two-week window around it, tends to cause unnecessary stress.
Using the default 28-day cycle when yours is different: If your cycles are consistently longer or shorter than 28 days, using the default skews your estimate by roughly however many days your cycle differs.
Ignoring an ultrasound date correction: Once your doctor confirms a due date from an early ultrasound, that date is generally more reliable than a recalculation from your last period.
Frequently Asked Questions
It's an estimate, not a guarantee. Only about 5% of babies are born on their exact due date, and most arrive within a week or two on either side. Your doctor may adjust the date later based on an early ultrasound, which is generally considered more accurate than a due date based on your last period alone.
If your cycles are irregular or you're not sure when your last period started, use the conception date method instead, or wait for your first ultrasound. Ultrasound measurements in the first trimester are typically accurate to within a few days.
The standard due date formula assumes a 28-day cycle with ovulation around day 14. If your cycles run longer or shorter than that, ovulation shifts too, which moves your actual conception date and, in turn, your due date. Entering your average cycle length adjusts for that.
A full-term pregnancy lasts about 266 days from conception, compared to 280 days from the first day of your last period. That 14-day gap accounts for the time between your period starting and ovulation actually happening.
The first trimester runs through week 13, the second from week 14 through week 27, and the third from week 28 until birth around week 40. Each trimester lines up with different stages of fetal development and different common symptoms.
An early ultrasound measures the size of the fetus (usually crown-rump length in the first trimester) and matches it to typical growth curves to estimate gestational age. If that measurement lands more than about 5 to 7 days away from a date calculated off your last period, most providers will shift your due date to match the ultrasound, since size-based dating tends to be more precise early on.
Since the embryo transfer date and the embryo's exact age are both known, IVF pregnancies can usually be dated more precisely than natural conception. A day 5 embryo is treated as 19 days past the start of the last period, and a day 3 embryo as 17 days past, so the due date works out to the transfer date plus roughly 261 or 263 days depending on which day the embryo was transferred.
These labels describe when birth happens relative to the due date rather than a single cutoff. Preterm is anything before 37 weeks, early term covers 37 to 38 weeks and 6 days, full term runs from 39 weeks to 40 weeks and 6 days, late term is 41 weeks, and postterm is 42 weeks or beyond. Babies born across the full-term window generally have similar outcomes, which is why doctors avoid inducing early without a medical reason.