How Your Due Date Is Calculated — And Why It Might Change

You've just found out you're pregnant and the first thing everyone asks is: when are you due? Your doctor gave you a date. The app on your phone gave you a slightly different one. An online calculator gave you a third date. So which one is actually right?

Here's how due dates work, why they change, and why that's completely normal.

The Standard Method: Last Menstrual Period (LMP)

The most common way to calculate a due date is called Naegele's Rule, and it's been the medical standard for over a century. You take the first day of your last menstrual period and add 280 days — or exactly 40 weeks.

Wait, 40 weeks? Pregnancy is 9 months, right?

Yes, but doctors measure from your last period, not from conception. Since most people ovulate around day 14 of their cycle, the first two weeks of "pregnancy" are counted before you actually conceived. So a 40-week pregnancy is roughly 38 weeks of actual fetal development.

Why Your Due Date Might Change After an Ultrasound

This happens often, and it's nothing to worry about. Here's what happens: an early ultrasound (usually done between 8-12 weeks) measures the baby's size — specifically the crown-to-rump length. From this measurement, the sonographer can estimate gestational age with pretty good accuracy.

If this measurement suggests a different due date than the LMP method — say, it's off by more than 5-7 days — the doctor will typically update the due date to match the ultrasound. The ultrasound is generally more accurate in early pregnancy.

What About Irregular Cycles?

Naegele's Rule assumes a 28-day cycle with ovulation on day 14. If your cycles are longer (say, 35 days), you likely ovulate later — around day 21 — which means your due date should be pushed back by about a week. Our pregnancy calculator lets you enter your actual cycle length to adjust for this.

IVF Due Dates Are Different

If you conceived through IVF, your due date is calculated differently — because you know exactly when fertilization happened. For a Day 5 blastocyst transfer, you add 261 days. For a Day 3 transfer, you add 263 days. There's no LMP guesswork involved.

How Accurate Are Due Dates?

Only about 5% of babies are born on their exact due date. Most arrive within a window of two weeks before to two weeks after. A "full-term" pregnancy is considered anywhere from 39 to 40 weeks and 6 days. Your due date is really the midpoint of a normal range, not a deadline.

Calculate Your Due Date

Use our pregnancy calculator to find your due date, see which week and trimester you're in, and track important milestones throughout your pregnancy.