Due Date & Pregnancy Week Calculator
＋More features: weeks and milestones from your due date
Even if you do not know your last period, you can work backward from the due date (40 weeks 0 days) your clinic gave you.
How many weeks and days today?
Milestone dates
A list of dates such as the second trimester, the start of prenatal maternity leave in Japan, the start of full term and the end of postnatal maternity leave.
Just enter the first day of your last period to find your due date and how many weeks and days pregnant you are now. It can also adjust for cycles other than 28 days. Weeks are counted from the last period, the standard method used by obstetricians in Japan — handy as a self-check before your clinic visit and before receiving the Maternal and Child Health Handbook (boshi techo) at your city office in Japan, or when thinking about when to tell your family.
How to use
- Pick the day your last period (the most recent one) started.
- Enter your cycle length if you know it (if it is not 28 days, the due date is adjusted).
- The due date, current week and trimester appear the moment you choose — no button needed.
How it is calculated
- Due date = first day of last period + 280 days (40 weeks 0 days of pregnancy). Counting pregnancy weeks from the first day of the last period is the standard obstetric method (Naegele's rule).
- If the cycle is not 28 days, (cycle − 28) days are added or subtracted to correct for the shifted ovulation (for example, a 35-day cycle moves the due date 7 days later).
- Pregnancy week = days elapsed from the starting point ÷ 7 (rounded down). The remainder is the “days” part.
- Trimesters: up to 13 weeks = first / 14–27 weeks = second / from 28 weeks = third
- Example: last period started January 1 (28-day cycle) → due date is October 8. As of July 24 you are 29 weeks 1 day pregnant (third trimester).
Sources (primary information)
- How pregnancy is counted (the due date is 40 weeks 0 days from the first day of the last period): Japan Society of Obstetrics and Gynecology, glossary on pregnancy and childbirth (in Japanese)
- The cycle adjustment is a common correction based on Naegele's rule.
Assumptions and notes
- If your cycle is irregular, this calculation can be off by more. The final due date is confirmed by a doctor using ultrasound.
- Only some babies arrive exactly on the due date; birth anywhere in the full-term range (37 weeks 0 days to 41 weeks 6 days) is common.
Frequently asked questions
Will the baby be born on the due date?
The due date is only the “40 weeks 0 days” reference day. Births from 37 weeks 0 days to 41 weeks 6 days are called full term, and most births fall in this range. It helps to allow about two weeks either side of the due date.
The due date my clinic gave me differs from this tool. Why?
Obstetricians often adjust the due date after measuring the baby's size (crown-rump length) in an early-pregnancy ultrasound. The doctor's due date is the correct one — use this tool as a rough guide before your visit.
How do the weeks relate to “month X of pregnancy” used in Japan?
In Japan, pregnancy is counted in months of 4 weeks: month 1 = weeks 0–3, month 2 = weeks 4–7, and so on. For example, 12 weeks is month 4 and 20 weeks is month 6.