Pregnancy Due Date Calculator
Calculate your estimated due date from the first day of your last menstrual period (LMP) or conception date.
Few dates are as eagerly anticipated as a baby's due date, and this calculator estimates it from the information your doctor will also use: the first day of your last menstrual period (LMP) or, if you know it, your conception date. It returns your estimated due date along with your current gestational age, giving you a clear timeline for the weeks and months ahead.
A due date is an estimate, not an appointment — only about one in twenty babies actually arrives on the predicted day, and a normal full-term birth can happen anywhere in a multi-week window around it. Still, the date is genuinely useful: it anchors prenatal appointments, screening windows, and your own planning. This tool gives you that anchor instantly, while making clear that the real schedule belongs to your baby and your care provider.
Plug in some numbers —
we'll crunch.
How to use
- 1Choose your calculation method: LMP (most common) or conception date.
- 2Enter the date — either the first day of your last period or your conception date.
- 3For LMP method: set your average cycle length if different from 28 days.
- 4See your estimated due date and key pregnancy milestones.
How it works
The standard method is Naegele's rule, which estimates a 40-week (280-day) pregnancy counted from the first day of your last menstrual period. The shortcut is: take the LMP date, add one year, subtract three months, and add seven days. This works because pregnancy is conventionally dated from the LMP rather than conception, which typically occurs about two weeks later.
If you know your conception or ovulation date instead, the due date is roughly 266 days (38 weeks) from there, since that skips the ~14 days between the LMP and ovulation. The rule assumes a regular 28-day cycle; if your cycles are consistently longer or shorter, the estimate shifts accordingly, which is why an early ultrasound — measuring the baby directly — is considered the most accurate dating method of all.
Worked examples
Estimating from the last period
Your last menstrual period began on March 1, 2026, with a typical 28-day cycle.
- Add one year: March 1, 2027.
- Subtract three months: December 1, 2026.
- Add seven days: December 8, 2026.
Your estimated due date is around December 8, 2026 — about 40 weeks from your last period. Expect a normal arrival window of roughly two weeks on either side.
Adjusting for a longer cycle
Same LMP, but your cycles run 35 days rather than 28.
- Naegele's rule assumes a 28-day cycle.
- A 35-day cycle means ovulation happens about a week later.
- Shift the estimate roughly seven days later.
The due date moves to around December 15, 2026. This is exactly the kind of adjustment an early dating ultrasound makes precisely, so always confirm the date with your provider.
Tips & common mistakes
Hold the date loosely. A due date marks the center of a normal birth window, not a deadline — full-term is defined as 37 to 42 weeks, and most babies arrive within that span rather than on the exact day. Treating the estimate as a target can create unnecessary anxiety as the day comes and goes.
An early ultrasound usually overrides the calendar calculation, especially if your cycles are irregular, because measuring the baby's size directly is more accurate than counting from a period date you may not remember precisely. If the dates disagree, your provider's ultrasound dating is the one to trust.
Use the timeline to plan prenatal care, screenings, and parental leave, but build in flexibility for an early or late arrival. Most importantly, this calculator is an informational estimate and not a substitute for medical care — attend your scheduled appointments and discuss any questions, symptoms, or concerns with your obstetrician, midwife, or another qualified healthcare professional, who can give advice specific to you and your baby.
Frequently asked questions
⚠ For informational purposes only. Your doctor's ultrasound-based date is more accurate. Always follow your healthcare provider's guidance.
Last reviewed: June 2026
About this calculator
Calculate your estimated due date from the first day of your last menstrual period (LMP) or conception date.