Baby Due Date Calculator

Your due date is really just an estimate built from averages, not a firm appointment on the calendar. Doctors have used the same basic math for over a century, and it still works about as well as anything more complicated. Here’s what that math actually is, and why your own date might land differently.

How Your Due Date Gets Calculated

The standard formula, called Naegele’s rule, takes the first day of your last period and adds 280 days, or 40 weeks. That’s not because pregnancy actually lasts 40 weeks from conception. Ovulation and fertilization usually happen around day 14 of a cycle, so a full term pregnancy is closer to 38 weeks from conception, but counting from your last period is the date most people actually remember, so that’s what the formula uses.

If you already had a dating ultrasound in your first trimester, your care provider’s estimate from that scan usually takes over from here. Early ultrasounds measure the size of the fetus directly, and they tend to be more accurate than LMP math alone, especially if your cycles run long, short, or irregular.

Why Only About 5 Percent of Babies Arrive On Their Due Date

A due date describes the middle of a range, not a single likely day. Full term now covers anywhere from 37 to 42 weeks, and healthy babies show up across that whole window all the time. First pregnancies also tend to run a few days past the estimated date more often than second or third ones do.

Think of your due date less like a deadline and more like the halfway point of about a five week window your baby is likely to arrive in.

What If My Cycle Isn’t Exactly 28 Days

The 280 day formula assumes a 28 day cycle with ovulation on day 14. If your cycles usually run longer or shorter than that, your actual due date can be off by several days from what a basic calculator like this one gives you. A longer cycle usually means you ovulate later, so your real due date is likely later than the estimate. A shorter cycle usually means the opposite.

This is exactly the kind of gap an early ultrasound is good at closing, since it measures your baby directly instead of relying on your cycle length.

A few quick questions

Can my due date change? Yes. It’s common for a due date to move by a few days after an early ultrasound, and that revised date is usually the one your provider goes with from then on.

What’s the difference between my due date and my conception date? Your due date is calculated from the first day of your last period. Your conception date is usually about two weeks later, whenever ovulation happened that cycle.

Is it normal to go past my due date? Very. Most first time parents deliver sometime in the two weeks after their estimated due date, and providers typically don’t start considering induction until closer to 41 or 42 weeks.