Pregnancy Due Date Calculator

This calculator estimates your due date using Naegele's Rule: the first day of your last menstrual period (LMP), plus 280 days, or 40 weeks. It's the same standard method used by most healthcare providers as a first estimate, based on the assumption of a typical 28-day menstrual cycle with ovulation occurring around day 14.

Alongside your estimated due date, the result also shows how far along you currently are — in completed weeks and days — and which trimester that falls in, using the standard boundaries of weeks 1–13 (first trimester), 14–27 (second trimester), and 28 onward (third trimester).

This is important to understand: this tool provides an estimate, not a medical diagnosis or a guaranteed birth date.Naegele's Rule assumes a regular 28-day cycle, so if your cycle is longer, shorter, or irregular, your actual due date may differ from this calculation by more than a few days. A first-trimester ultrasound, performed by a healthcare provider, is generally considered more accurate than an LMP-based estimate and should take priority if the two disagree. Use this calculator as a general planning reference, and confirm your due date with a doctor or midwife.

A worked example

Say the first day of your last menstrual period was March 1. Naegele's Rule adds 280 days (40 weeks) to that date: March 1 plus 280 days lands on December 6 — the estimated due date this calculator would show. If today's date is May 10, that's 70 days after March 1 — exactly 10 completed weeks and 0 extra days of gestational age, placing you in the first trimester (weeks 1–13), with 210 days (30 weeks) remaining until the December 6 estimate.

A common misconception

A due date reads like a deadline, but it isn't one. Mayo Clinic describes it plainly: the due date is simply the date on which you'll be 40 weeks pregnant, not a prediction of the exact day labor will start — few pregnancies end precisely on that day, and a birth anywhere within a few weeks of it is entirely normal. It's also worth knowing that ACOG's own guidance on this topic is titled "Methods for Estimating the Due Date" — plural — because Naegele's Rule (LMP-based) is one of several accepted methods, alongside first-trimester ultrasound dating and, for pregnancies conceived via assisted reproductive technology, the known transfer date; ACOG recommends using whichever is most accurate for a given pregnancy, not always defaulting to LMP.

When Naegele's Rule doesn't apply well

This calculator, like Naegele's Rule itself, is built for a single typical pregnancy with a regular 28-day cycle — it doesn't apply well to twin or multiple pregnancies, which reliably run shorter than a singleton's 40 weeks. ACOG guidance points to delivery around 38 weeks for uncomplicated dichorionic twin pregnancies, and around 36–37 weeks for monochorionic twin pregnancies — well before this calculator's 280-day estimate. A due date calculated this way for a twin pregnancy should be understood as a singleton-style reference point, not the expected delivery timing, which a doctor manages separately based on the specific pregnancy.

What to actually do with this date

Use this estimate to orient early planning — roughly which trimester you're in, when to schedule early prenatal visits, when parental leave or travel plans might need to work around. Per ACOG's own committee opinion on due date methods, a first-trimester ultrasound is considered the most accurate way to confirm or revise gestational age, accurate to within about 6 days; once your provider gives you a confirmed due date, that figure should take priority over this calculator's LMP-based estimate.

LMP dating vs. ultrasound dating

LMP-based dating — what this calculator does — only needs a date you likely already know, which makes it useful before any clinical visit has happened, but it assumes a textbook cycle and is only as accurate as your memory of when your last period actually started. Ultrasound dating measures the embryo or fetus directly and doesn't depend on cycle regularity or recall at all, which is why ACOG considers a first-trimester ultrasound the more accurate method whenever the two disagree. Use this calculator as a starting estimate before that appointment, not as a substitute for it.