Pregnancy Due Date Calculator

You only need two pieces of information, both usually available in any period-tracking app:

1. Last menstrual period (LMP): the first day of bleeding of your most recent period — not the day it ended. Enter day, month and year separately. 2. Average cycle length: the number of days from the first day of one period to the first day of the next. Most cycles run between 21 and 35 days; if you have never tracked yours, keep the default of 28.

The calculator returns the day, month and year of your estimated due date — the theoretical end of week 40 of pregnancy. Do not skip the cycle adjustment if your cycles are long or short: a 35-day cycle pushes the date a full week later, which changes the gestational age you would quote when scheduling a first-trimester ultrasound or an 11-to-13-week screening with your provider.

A practical tip: if you are unsure between two possible LMP dates, run both. Each day of difference in the LMP shifts the due date by exactly one day, so the uncertainty you introduce is small compared with the natural spread of labor itself — full term is anywhere from 37 to 42 weeks, a five-week window.

Naegele's rule, attributed to the 19th-century German obstetrician Franz Naegele, is still the textbook starting point for pregnancy dating. In its cycle-adjusted form:

Due date = LMP + 280 days + (cycle length − 28)

The 280 days are 40 weeks counted from the last menstrual period — gestational age is measured from the LMP even though conception typically happens about two weeks later. The correction term (cycle − 28) exists because the part of the cycle that varies between people is the pre-ovulation phase: in a 35-day cycle, ovulation usually comes 7 days later, so the pregnancy effectively starts 7 days later than the classic rule assumes.

Worked numeric example. LMP on June 15, 2026, with a 30-day cycle:

• Days to add: 280 + (30 − 28) = 282
• June 15, 2026 + 282 days = March 24, 2027

Under the hood, the calculator converts the calendar date into a running day number (a proleptic Gregorian calendar algorithm, the same math used by date libraries), adds the days, and converts back to day/month/year. Leap years and month boundaries are handled exactly — no 30-day-month approximations.

Sarah, in Austin, has tracked her cycle for a year with an app: it averages 25 days, shorter than the textbook 28. Her last period started on January 4, 2026, and she wants an estimate before her first prenatal visit.

Step 1 — total days to add: 280 + (25 − 28) = 277 days.

Step 2 — add 277 days to January 4, 2026. January contributes its remaining 27 days, then February 28 (2026 is not a leap year), March 31, April 30, May 31, June 30, July 31, August 31, September 30 — landing on October 8, 2026.

Result: October 8, 2026.

Had Sarah used the unadjusted classic rule (assuming a 28-day cycle), the estimate would have been October 11 — three days later. Three days sounds trivial, but at the 11-to-13-week mark it can decide whether a nuchal translucency scan is booked inside or outside its valid window, and near the end of pregnancy it shifts the date when conversations about post-term monitoring would begin. That is exactly why the cycle-length input is worth filling in with real tracked data instead of the default.

• Entering the last day of your period instead of the first. The LMP is always the first day of bleeding. Mixing them up adds a systematic 3-to-6-day error.
• Typing a date that does not exist (April 31, for instance). The calculator rolls it into the next real day (May 1), silently shifting the result — double-check the date is valid.
• Mistaking implantation spotting for a period. Light mid-cycle spotting is not a menstrual period; using it as the LMP pulls the due date about two weeks too early.
• Highly irregular cycles. If your cycle length swings more than 7-8 days month to month, the average stops being meaningful and Naegele's rule loses reliability; first-trimester ultrasound dating becomes the reference instead.
• IVF and assisted reproduction. After an embryo transfer, dating starts from the transfer date (minus 3 or 5 days depending on embryo stage), not from a menstrual period that may not exist. This calculator does not apply.
• Reading the date as an appointment. Only about 4-5% of births happen on the exact estimated date; full term spans weeks 37 through 42.

This tool is educational and for orientation only. It is not medical advice and does not replace evaluation by your OB-GYN, midwife or family physician — the official dating of your pregnancy is set by your healthcare professional, usually with a first-trimester ultrasound.