Due date calculator: how pregnancy due dates are estimated

An estimated due date gives expecting parents a target to plan around, but many do not realize how the date is calculated or how accurate it really is. Here is what you need to know about due dates.

Naegele’s rule explained

The standard method for calculating a due date is Naegele’s rule. Take the first day of your last menstrual period (LMP), add 280 days (40 weeks), and that is your estimated due date. The formula assumes a 28-day cycle with ovulation on day 14.

For example, if your LMP was January 1, adding 280 days gives you October 8. A simpler version is to add 9 months and 7 days. January 1 plus 9 months is October 1, plus 7 days is October 8.

Why due dates are estimates

Only about 4 percent of babies are born on their exact due date. About 80 percent are born within two weeks before or after the due date. Full-term pregnancy is defined as 37 to 42 weeks, giving a five-week window that is all considered normal.

Ovulation does not always happen on day 14. Cycle length varies, and ovulation timing varies even within regular cycles. This alone can shift the true due date by several days to a week.

Conception date is rarely known exactly unless pregnancy is achieved through fertility treatments. Even then, implantation timing adds variability.

Ultrasound dating

First-trimester ultrasound dating is more accurate than LMP-based calculation. Between 8 and 13 weeks, measuring the crown-rump length of the embryo estimates gestational age within about 5 to 7 days. Later ultrasounds are less accurate for dating because fetal growth rates vary more in the second and third trimesters.

When ultrasound dating and LMP dating differ by more than 7 days in the first trimester, the ultrasound date is typically used. This is standard practice in most obstetric guidelines.

Factors that affect delivery timing

First-time pregnancies tend to go slightly longer on average than subsequent pregnancies. Multiples (twins, triplets) almost always arrive early — the average twin pregnancy delivers at 36 weeks.

Maternal age, pre-existing health conditions, and pregnancy complications can all shift delivery timing. Inductions and scheduled C-sections account for about 30 percent of deliveries in the US, which means many due dates are planned rather than natural.

Using a due date calculator

Most due date calculators let you enter either your LMP or your conception date if known. Some also incorporate cycle length for more accuracy. If you have a 35-day cycle, ovulation happens around day 21 instead of day 14, which shifts the due date by a week.

Use the Due Date Calculator to get your estimate and understand the range of possible delivery dates. Remember that the due date is a milestone, not a deadline. Your healthcare provider will adjust the date based on early ultrasound and clinical assessment as your pregnancy progresses.

Planning around the estimate

Use the due date as a planning tool, not an expectation. Pack your hospital bag by 36 weeks, finalize parental leave arrangements for a range of dates, and mentally prepare for the possibility of going past 40 weeks. Most first-time mothers deliver between 40 and 41 weeks. The Due Date Calculator gives you the starting point for all that planning.