Pregnancy Due Date Calculator

How Naegele's rule actually works

Franz Naegele published the formula in 1812: first day of the last menstrual period (LMP) plus 280 days, or equivalently LMP plus 9 months and 7 days. The math assumes a textbook 28-day cycle with ovulation on day 14, and that fertilization happened that same day. From that fertilization point, term pregnancy averages 266 days.

That stack of assumptions is where the rule starts to drift. Roughly 30% of women have cycles shorter or longer than 28 days. Ovulation day varies by cycle, sometimes by a week. The 14-day "luteal phase" between ovulation and the next period is one of the few constants — but the follicular phase (period to ovulation) can stretch from 10 to 25 days in a normal, fertile woman. The formula collapses that variability into a single date with no error bar attached.

Why ultrasound dating is more accurate than LMP

The American College of Obstetricians and Gynecologists (ACOG) recommends that a first-trimester ultrasound between 8 and 13 weeks supersede the LMP-based due date when the two disagree by more than 5 to 7 days. A crown-rump-length (CRL) measurement at 8 to 13 weeks is accurate to within 5 days of true gestational age in about 95% of pregnancies. After 14 weeks, the error band widens to 10 to 14 days. After 22 weeks, ultrasound dating is poor enough that it rarely overrides the LMP date.

If you have an irregular cycle, are unsure of your LMP, were on hormonal birth control within 3 months of conceiving, or conceived while breastfeeding, the LMP-based date is essentially a guess. The 8-to-13-week ultrasound is the single most important dating event in pregnancy. Ask for it explicitly if it isn't scheduled by 10 weeks.

What a due date actually predicts

Only about 5% of babies are born on their actual due date. The clinical reality of full-term births:

• About 50% of babies arrive within 1 week before or after the due date
• About 80% arrive within 2 weeks on either side
• "Term" pregnancy covers 37 weeks 0 days through 42 weeks 0 days
• First-time mothers tend to deliver 4 to 8 days past their due date on average
• Mothers who've had a previous full-term birth tend closer to the LMP-based date

The due date is a midpoint, not a target. Treat any number you get from a calculator as the peak of a probability curve that spreads roughly four weeks wide.

Common mistakes and misconceptions

The biggest misconception is that "going past 40 weeks" is unusual or risky on its own. For a first pregnancy with no complications, delivering at 40 weeks 5 days is statistically normal. ACOG defines "late term" as 41 weeks 0 days through 41 weeks 6 days and "postterm" as 42 weeks and beyond — that's when monitoring intensifies and induction is typically offered. The 2018 ARRIVE trial showed that elective induction at 39 weeks for low-risk first-time mothers slightly reduced cesarean rates, which is now driving more 39-week inductions in the US. Talk to your OB about what their practice recommends and why.

The second misconception: conception-date math. If you know the exact date of conception (single-sex day, IVF transfer, IUI), the math is conception + 266 days, not LMP + 280 days. A conception-date calculator should subtract 14 days from the implied LMP before applying the 280-day rule.

How to use this tool

Pick LMP if you have a regular cycle and remember when your last period started. Pick conception if you know the exact day. Pick IVF transfer if you went through IVF — that path uses different math, and the IVF calculator is a separate tool. The result is the LMP-based due date, your current week and trimester, and the dates of your major milestones (end of first trimester, anatomy scan window, viability, glucose screen, full term). Save the date but treat it as a target month, not a target day.

The induction debate at 41 vs 42 weeks

For decades, the standard was to let labor start spontaneously up to 42 weeks before inducing. The ARRIVE trial (NEJM, 2018) changed the conversation. The study randomized 6,106 low-risk first-time mothers to either elective induction at 39 weeks or expectant management. The 39-week-induction group had a cesarean rate of 18.6% versus 22.2% in the expectant-management group, with no increase in adverse outcomes for babies. Many US hospitals now offer routine 39-week induction for low-risk first pregnancies.

What that doesn't mean: every pregnancy should be induced at 39 weeks. ARRIVE applies to a specific population — uncomplicated first pregnancies at academic medical centers. If your pregnancy is uncomplicated and you've delivered before, the math is different. If you have gestational diabetes, hypertension, advanced maternal age, or other risk factors, induction timing is individualized. The conversation to have with your OB by 38 weeks: what's their recommendation given your specific situation, and why.

When to call your OB

Schedule your first prenatal appointment by 8 to 10 weeks so the dating ultrasound can happen in the accurate window. Call your provider sooner than that scheduled appointment if you have heavy bleeding, severe one-sided cramping, fever over 100.4 F, or persistent vomiting that prevents keeping fluids down. After 41 weeks, you should be in active conversation with your OB about monitoring (non-stress tests, biophysical profiles), membrane sweeps, and induction timing. The exact threshold for induction varies by practice and by individual risk factors. The 41-versus-42-week discussion is one to have with your specific provider, not a calculator.