Due Date Calculator
Estimate your baby's due date from your last menstrual period, or work backward from a due date.
About this calculator
This calculator uses Naegele's rule, the standard method used by most healthcare providers, which estimates a due date as 280 days (40 weeks) from the first day of your last menstrual period (LMP). It also works in reverse: enter a due date to estimate the corresponding LMP. This is an estimate based on average cycle length; only about 5% of babies are actually born on their exact due date.
Naegele's rule is often written as a shortcut: take the first day of your last period, add one year, subtract three months, and add seven days. It dates back to the early 1800s and assumes a 28-day cycle with ovulation around day 14. If your cycles are usually longer, you likely ovulated later, and your due date may be a little later than the LMP method suggests; shorter cycles shift it earlier.
Many providers confirm or adjust the due date with an ultrasound in the first trimester, when measurements of the embryo are most consistent from one pregnancy to the next. If the ultrasound date and the LMP date differ by more than a small margin, the ultrasound date is usually used for the rest of the pregnancy. For pregnancies conceived through IVF, the due date is calculated from the embryo transfer date instead, which is known precisely.
A due date is best thought of as the middle of a range. Babies born anywhere from 39 weeks through 40 weeks and 6 days are considered full term, and it's common to deliver a week or more before or after the date itself.
Frequently asked questions
- How accurate is a due date calculated from LMP?
- It's an estimate based on a standard 28-day cycle with ovulation on day 14. Actual cycle length varies, so an early ultrasound is generally considered more accurate than LMP-based dating alone; your provider may adjust your due date based on ultrasound measurements.
- What if I don't remember my last menstrual period exactly?
- Use your best estimate. If you know your due date from an ultrasound instead, use the reverse mode to work backward; that will generally be more accurate than a remembered LMP date.
- Why 280 days?
- 280 days (40 weeks) is counted from the first day of your last period, not from conception, even though conception typically happens about two weeks after that. This is the standard convention used in prenatal care.
- Can my due date change?
- Yes. It's most commonly adjusted after an early ultrasound if the measurements suggest a different date than your LMP. Once a due date has been set from an early ultrasound, providers generally don't change it based on later scans, because later measurements vary more with the individual baby's growth.
- What does "full term" mean?
- The American College of Obstetricians and Gynecologists defines early term as 37 weeks through 38 weeks 6 days, full term as 39 weeks through 40 weeks 6 days, late term as 41 weeks through 41 weeks 6 days, and post-term as 42 weeks or later. Births before 37 weeks are considered preterm.