Pregnancy Due Date Calculator

How to use the pregnancy due date calculator

Enter a date in "Reference date", choose whether it is your last menstrual period, conception date or ovulation date in "Reference type", and the estimated due date plus how many weeks pregnant you are today, the pregnancy month and the trimester appear instantly. It recalculates whenever you change a value.

• Last menstrual period (LMP): the most common reference. The due date is the first day of your last period plus 280 days (40 weeks).
• Conception / ovulation date: these occur roughly two weeks after the last period, so 266 days are added to reach the due date.
• Cycle length: used only in last-menstrual-period mode. If it differs from 28, the due date is shifted by that difference (a longer cycle pushes the due date later, a shorter one earlier).
• Pregnancy weeks: counted from the first day of the last menstrual period as "0 weeks 0 days". The due date is exactly 40 weeks 0 days. The month shown is a 4-week (lunar) month.

Handy for

• Getting a rough estimate of your due date
• Quickly checking "how many weeks and days pregnant am I now?"
• Seeing the approximate pregnancy month and trimester (1st–3rd)
• Seeing how a cycle other than 28 days shifts the due date

Medical note: This tool is an estimate based on Naegele's rule, not a medical diagnosis or advice. Your actual due date is determined by a clinic, for example from an ultrasound scan. If you have any concerns about the progress of your pregnancy or your health, do not rely on this tool alone — please consult an obstetrician or other clinic.

Difference from a Clinical Due Date

The conventional 280 days from the last menstrual period is not the whole clinical dating process. ACOG's Methods for Estimating the Due Date describes that convention while identifying first-trimester ultrasound as the most accurate way to establish or confirm gestational age (reaffirmed 2025; checked July 14, 2026).

Irregular cycles, uncertain LMP recall, variable ovulation, fertility treatment, and embryo transfer can make a simple adjustment inappropriate. This tool does not diagnose pregnancy or determine appointments, tests, or medication timing. Use the due date assigned by the treating clinician when available.

Seek medical advice for bleeding, severe pain, or other concerning symptoms regardless of the calculated date. Use the Date Calculator for ordinary date intervals.

Pregnancy weeks and months at a glance

Counting starts at the first day of the last menstrual period, which is week 0 day 0; the due date is week 40 day 0, that is 280 days later. The Japanese "pregnancy month" is weeks ÷ 4 + 1. Trimesters break at week 14 and week 28.

Because the Japanese month is weeks ÷ 4 + 1, month 10 spans weeks 36–39 and month 11 spans weeks 40–43; the due date at week 40 day 0 is the first day of month 11. Note that this is the Japanese convention and does not match the colloquial "nine months" — read the gestational week as the primary figure. Term birth is weeks 37–41, so delivering during month 11 is entirely normal. These figures follow Naegele's rule and are an estimate. A cycle other than 28 days shifts the result by the difference, and a conception or ovulation date adds 266 days instead. Real due dates are set by a clinician using ultrasound. Past 42 weeks (294 days), seek medical advice.

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