Enter the dates
How to use the due date calculator, and how pregnancy weeks are counted
When a pregnancy is confirmed, the first questions are the due date and "how many weeks am I now?". From the first day of your last menstrual period (and your cycle length) this tool shows the estimated due date, the gestational age on any reference date, and the trimester — a useful starting point when planning check-ups or parental leave.
How to use it
Enter the first day of your last menstrual period. If your cycle is not 28 days, entering the cycle length applies a correction. Changing the reference date lets you check "how many weeks was I at the last appointment?" or "what week is a month before the due date?".
How the due date is estimated (Naegele's rule)
Clinics generally use Naegele's rule: add 280 days (40 weeks) to the first day of the last period. The shorthand version is to subtract three months and add seven days. A cycle longer than 28 days usually means later ovulation, which pushes the due date back accordingly.
Week 0 day 0 is the first day of your last period
Counter-intuitively, gestational age is counted from the first day of the last menstrual period, not from conception. Ovulation and fertilisation happen roughly two weeks later, so at "5 weeks pregnant" it is about three weeks since conception. Under this convention the due date falls at exactly 40 weeks and 0 days.
Trimesters and the term window
Up to 13 weeks 6 days is generally the first trimester, 14 weeks 0 days to 27 weeks 6 days the second, and 28 weeks onward the third. Birth between 37 weeks 0 days and 41 weeks 6 days is considered term. The due date is therefore the centre of a window rather than a deadline: only a few per cent of babies arrive on it, with the rest spread over the surrounding weeks.
Always follow your clinician
This estimate assumes a regular menstrual cycle. Irregular cycles, unusual ovulation timing and assisted conception can all shift the real date substantially. The authoritative due date and assessment of a pregnancy come from a clinician, based on ultrasound and examination. Do not use this page for medical decisions. For plain day counting, see the date calculator.
Frequently asked questions
How is the due date calculated?+
If the cycle differs from 28 days, it is adjusted by the difference. It is a general estimate.
When does pregnancy week counting start?+
Actual ovulation and conception occur about two weeks later, but weeks are conventionally counted from this point.
What if my cycle is not 28 days?+
A longer cycle shifts it later, a shorter one earlier. Still an estimate.
How accurate is the due date?+
Always follow your doctor.
Are my inputs stored?+
・Due date = first day of last menstrual period + 280 days (40 weeks), adjusted by (cycle − 28)
・Gestational age = (reference date − first day of last period) ÷ 7, counting that day as week 0 day 0
・Trimesters (general): first up to 13w6d / second 14w0d–27w6d / third from 28w0d
・Based on Naegele's rule. It does not hold for irregular cycles or shifted ovulation.
・Japan Society of Obstetrics and Gynecology, glossary of obstetric and gynecological terms: term birth spans 37w0d to 41w6d
・The authoritative due date and assessment of a pregnancy are determined by a clinician using ultrasound and examination. Do not use this page for medical decisions.
This site does not represent any medical institution and does not guarantee the accuracy of the results. The due date and pregnancy week here are a general estimate and may differ from reality. For the actual due date, pregnancy progress and health management, always follow the diagnosis of a doctor or medical institution.