Pregnancy Due Date Calculator

Pregnancy due dates are almost always estimated the same way: by counting 280 days, or 40 weeks, from the first day of your last menstrual period (LMP), a method known as Naegele's rule. It's a bit of a historical quirk that pregnancy is measured from before conception actually happens, but it's used this way because the exact date of conception is rarely known with certainty, while the start of the last period usually is.

This calculator takes the date you enter and adds 280 days to estimate your due date, then compares today's date to your LMP to calculate how far along you currently are, in completed weeks and days. That "current week" number is the same one your doctor or midwife will reference at appointments, and it's what determines which trimester you're in: the first trimester runs through week 13, the second through week 27, and the third from week 28 until birth.

It's worth remembering that this method assumes a textbook 28-day cycle with ovulation around day 14, which doesn't match everyone. Shorter or longer cycles shift the actual conception date earlier or later than the standard assumption, which is one reason an early ultrasound, which measures the size of the developing fetus directly, is often considered more accurate than an LMP-based estimate, especially when cycles are irregular or the LMP date isn't well remembered.

Only about 1 in 20 babies actually arrives on their exact due date — most arrive within a couple of weeks before or after it. Think of this date less as a deadline and more as the midpoint of a normal, several-week window, and use it mainly to track milestones and schedule prenatal care rather than to predict an exact birthday.