Counting Pregnancy Weeks and Months: Why It Starts at 0 Weeks 0 Days, with a Chart (Japanese System)

Pregnancy weeks start at "0 weeks 0 days"

Pregnancy weeks are counted with the first day of your last menstrual period as "0 weeks 0 days". Each week runs from day 0 to day 6, and on the seventh day it becomes day 0 of the next week. It works like counting age in completed years: just as a newborn is 0 years old, the first week is week 0.

"4 weeks pregnant" refers to the whole seven days from 4 weeks 0 days to 4 weeks 6 days. Notations such as "12w3d" and "12+3" both mean 12 weeks 3 days.

How to work out how many weeks and days you are today

1. 1

   Count the days from the first day of your last period to today

   If your last period started on August 3, 2026 and today is September 24, 2026, August 3 to 31 is 28 days and September 1 to 24 is 24 days, for a total of 52 days.

2. 2

   Divide the days by 7

   52 ÷ 7 = 7 remainder 3.

3. 3

   The quotient is weeks, the remainder days

   You're 7 weeks 3 days pregnant. That's pregnancy month 2 (weeks 4–7) in Japan's count, in the early stage.

The easiest mistake when calculating by hand is the number of days when crossing months. Miscounting the last day of a month (30 or 31, or 28 or 29 in February) throws you off by a day or two. Weekdays make it easy to check: if your last period started on a Monday, every Monday is "X weeks 0 days", so if today is Thursday, you're "X weeks 3 days".

In Japan, a pregnancy month is four weeks

In Japan, pregnancy weeks are grouped into blocks of four (28 days) called "pregnancy month X". Weeks 0–3 are pregnancy month 1, weeks 4–7 month 2, and so on up to weeks 36–39, month 10 (the final month). Unlike a calendar month (28–31 days), the key point is that it always advances in steps of exactly 28 days.

10 months × 28 days = 280 days, so the day after pregnancy month 10 ends is exactly the due date (40 weeks 0 days). There's an old Japanese saying, totsuki tōka ("ten months and ten days"), but on the calendar the due date comes about 9 months and a week after your last period. "Pregnancy month 10" and "10 calendar months later" are different things, so be careful when counting on a calendar.

Where the early, middle and late stages divide

In the terminology of the Japan Society of Obstetrics and Gynecology, pregnancy is split into three stages. The so-called "stable period" usually refers to 16 weeks 0 days, the start of the middle stage, and the "final month" is pregnancy month 10 (from 36 weeks 0 days).

The divisions line up exactly with the months, so you can also remember them as "middle stage from month 5" and "late stage from month 8". What happens in each stage and how the checkup interval changes are summarized in Pregnancy schedule by week.

Counts that are easy to confuse

• "Pregnancy month 3" and "the 3rd month of pregnancy": in Japanese usage both mean weeks 8–11. Neither means "three months have passed".
• Pregnancy weeks vs fetal age: the baby's age counted from fertilization (fetal age) is two weeks less than the pregnancy week. At 10 weeks 0 days, the fetal age is 8 weeks 0 days.
• Age after birth: once born, the count switches to "X months old", counted in calendar months from the birthday as day 0, no longer in four-week blocks.
• Counting outside Japan: "X weeks pregnant" in English uses the same count as Japanese pregnancy weeks. Months, however, are usually counted closer to calendar months and differ from Japan's pregnancy months.

Checking each week's dates on a calendar

The Pregnancy week calendar tab of the Due Date & Pregnancy Week Calculator lists, for each week from 0 to 41, which dates it covers, which pregnancy month and stage it falls in, and the typical checkup interval. The week containing today is highlighted, so you can see your current week at once without doing the calculation above by hand.

A quick-reference chart of pregnancy months appears below it, which also helps when filling in your maternal and child health handbook or telling your workplace which month you're in. To find out which week a particular date falls in, the Date ⇔ week tab is handy.

FAQ

When do pregnancy weeks start?

The first day of your last period is counted as 0 weeks 0 days. Because the count includes time before you were actually pregnant, ovulation and fertilization fall around 2 weeks 0 days, your next expected period at 4 weeks 0 days and the due date at 40 weeks 0 days. If your clinic revises the due date, recount with 280 days before the due date as 0 weeks 0 days.

Which weeks are pregnancy month 1?

Pregnancy month 1 runs from 0 weeks 0 days to 3 weeks 6 days. You almost never know you're pregnant yet at this point; most people notice when their period is late, after entering month 2 (weeks 4–7).

What month is 16 weeks pregnant?

16 weeks is pregnancy month 5. In Japan a pregnancy month is counted as four weeks, so weeks 16–19 are month 5. From 16 weeks 0 days you're in the middle stage, the so-called stable period.

I'm in pregnancy month 10, but only nine months have passed since my last period.

That's because a pregnancy month is counted as 28 days. Calendar months have 28 to 31 days, so pregnancy month 10 (36 weeks 0 days) begins about 8 months and 10 days after your last period on the calendar, and the due date (40 weeks 0 days) is about 9 months and a week after.