How do I calculate a due date?

Add 280 days (40 weeks) to the first day of your last period. If your last period started on January 1, 2026, your due date is October 8, 2026. That assumes a 28-day cycle; with a 35-day cycle the due date is 7 days later, October 15.

What is Naegele's rule?

It is a mental-math method: take the first day of your last period, subtract 3 from the month (or add 9 if you cannot subtract), and add 7 to the day. A last period starting January 1 gives October 8. Because it ignores differences in month length, it can differ by a few days from counting 280 days. For example, with a last period on May 31, 2026, the rule gives March 10, 2027, while counting 280 days gives March 7. This tool shows both.

How do I find out what week and day of pregnancy I am in today?

Treat the first day of your last period as 0 weeks 0 days, and divide the number of days up to today by 7: the quotient is the weeks and the remainder is the days. With a last period on June 1, 2026, September 23, 2026 is day 114 (16 weeks 2 days), so you are 16 weeks 2 days pregnant (month 5, second trimester). This tool shows today's week as soon as you pick a date.

Which weeks make up each month of pregnancy?

Each month is counted as 4 weeks. Month 1 is weeks 0-3, month 2 is weeks 4-7, month 3 is weeks 8-11, month 4 is weeks 12-15, month 5 is weeks 16-19, month 6 is weeks 20-23, month 7 is weeks 24-27, month 8 is weeks 28-31, month 9 is weeks 32-35, and month 10 (the final month) is weeks 36-39. Week 40 is the due date.

Does the due date change if my cycle is not 28 days?

Yes. The 280-day calculation assumes a 28-day cycle (ovulation on day 14 after the last period), so people with longer cycles ovulate later and the due date shifts later too: 7 days later for a 35-day cycle, 2 days later for a 30-day cycle. If your cycle is irregular the error can be large, so give priority to the due date set by an early-pregnancy ultrasound.

Can I calculate a due date from an ovulation date or an IVF transfer date?

Yes. The ovulation (conception) date is 2 weeks 0 days, so add 266 days to it to get the due date. For embryo transfers it depends on the embryo's age: add 261 days to the transfer date for a blastocyst (day-5 embryo) or 263 days for an early embryo (day-3). A blastocyst transferred on January 20, 2026 gives a due date of October 8, 2026.

When do full term, the final month, and the second trimester start?

The second trimester (often called the stable period in Japan) starts at 16w0d, the final month (month 10) at 36w0d, and full term runs from 37w0d to 41w6d. With a last period on June 1, 2026 (due date March 8, 2027), the second trimester starts September 21, 2026, the final month on February 8, 2027, and full term runs February 15 to March 21, 2027.

What are preterm and post-term births in terms of weeks?

Births from 22w0d to 36w6d are preterm, 37w0d to 41w6d are full term, and 42w0d onward are post-term. The "Date ⇔ week" tab shows which category a birth on the selected date would fall into, plus the dates of each category for this due date.

Can I calculate when I will reach week 36?

Yes. On the "Date ⇔ week" tab, enter "36" or "36w2d" under "When do I reach pregnancy week...?" and the date appears. With a last period on June 1, 2026, 36w0d is February 8, 2027. Conversely, pick the date of an appointment or a trip and it shows which week and day that is.

When can I start maternity (prenatal) leave?

You can start 6 weeks before the due date, counting the due date (14 weeks before for twins or more). With a due date of October 1, 2026, prenatal leave starts August 21, 2026 (June 26 for multiples). Postnatal leave runs 8 weeks from the day after birth, so for a birth on the due date it ends November 26.

My due date is in early April. Will my child be early-born?

A child born by April 1 is early-born and enters the earlier school year; a child born on April 2 or later is in the next year. For example, someone born April 1, 2027 enters elementary school in April 2033, and someone born April 2 enters in April 2034. If the April 1/2 boundary falls within two weeks either side of your due date, the tool shows a notice.

My due date differs from what my clinic told me.

In early pregnancy (around weeks 8-11) an ultrasound measures the baby's crown-rump length (CRL), and if it differs from the date based on your last period by about a week or more, the due date is usually corrected to the ultrasound result. Once your clinic sets a due date, choose "From due date" so the weeks and calendar match your clinic's counting.

Due Date Calculator & Pregnancy Weeks (What Week Am I Today?)

Enter the first day of your last period and instantly see your due date, today's gestational age (weeks and days), pregnancy month, and whether you are in the first, second, or third trimester. It also handles irregular cycle lengths, calculation from an ovulation date or IVF embryo transfer date, and counting back from a due date set by your clinic. A pregnancy week calendar shows the dates each week starts and ends, two-way conversion answers "What week is this date?" and "When do I reach week 36?", and the same page shows your prenatal leave start date and your child's school year (early-born or not).

ABOUT

About Due Date Calculator & Pregnancy Weeks (What Week Am I Today?)

The Due Date & Pregnancy Week Calculator works out your due date (40 weeks 0 days of pregnancy) from nothing more than the first day of your last period, and tells you what week and day of pregnancy you are in today, which month of pregnancy that is, and whether you are in the first, second, or third trimester. It corrects for cycles that are not 28 days, and also works from an ovulation date, an intrauterine insemination or timed intercourse date, an IVF/ICSI embryo transfer date (early embryo or blastocyst), or a due date your clinic has already given you. On one page you can also check a pregnancy week calendar showing which dates each week covers, a pregnancy month chart, two-way conversion for "What week is this date?" and "When do I reach week 36?", the dates of the second trimester, the final month and full term, when you can start prenatal leave, and your child's school year (whether they would be an early-born child in Japan).

How to use Due Date Calculator & Pregnancy Weeks (What Week Am I Today?)

1Under "What do you want to calculate from?", choose whichever you know: last period, ovulation date, embryo transfer date, or due date
2Pick that date on the calendar (for a last period, the first day of bleeding; if your cycle is not 28 days, also enter your cycle length)
3On the "Due date & today's week" tab, check your due date, today's gestational age, and pregnancy month
4In "Upcoming milestones" below, see dates such as heartbeat confirmation, the second trimester, the final month, and full term
5On the "Pregnancy week calendar" tab, check which dates each week and each month cover
6On the "Date ⇔ week" tab, find out which week an appointment or trip date falls in, or when you reach a given week

Great for

Calculate a due date from the last period after a positive pregnancy test
Find out what week and day of pregnancy you are in today, and how many months
Adjust the due date to a cycle that is longer (or shorter) than average
Calculate the due date and current week from a blastocyst transfer date in IVF
Build a calendar of when each week starts from the due date your clinic gave you
Check which week of pregnancy a trip or a return-to-parents'-home date falls in
Find the date you reach week 36 (final month) or week 37 (full term)
Work out from your due date when you can start prenatal leave
Check whether a due date in late March or early April means an early-born school year in Japan

Good to know

The due date is day 280 (40 weeks 0 days) counting the first day of your last period as 0 weeks 0 days. If your cycle is longer (shorter) than 28 days, ovulation is assumed to be later (earlier), and the due date is shifted later (earlier) by that many days.
The ovulation date (conception date) is counted as 2 weeks 0 days. For an embryo transfer, the egg retrieval date is 2 weeks 0 days and the transfer date is later by the embryo's age (a blastocyst, i.e. a day-5 embryo, is 2 weeks 5 days; an early embryo, i.e. a day-3 embryo, is 2 weeks 3 days).
Pregnancy weeks are counted as weeks and days, and one week spans days 0 to 6. Pregnancy months use 4 weeks (28 days) per month: weeks 0-3 are month 1 and weeks 36-39 are month 10 (the final month).
The stages follow the terminology of the Japan Society of Obstetrics and Gynecology (first trimester: up to 15w6d, second: 16w0d-27w6d, third: from 28w0d; preterm: 22w0d-36w6d, full term: 37w0d-41w6d, post-term: from 42w0d).
The timing of the heartbeat check, morning sickness, and fetal movement are general guides and vary from person to person.
An early-pregnancy ultrasound (around weeks 8-11) measures the baby's size (crown-rump length) and may correct the due date. Once your clinic sets a due date, choose "From due date" and recalculate.
Under Article 65 of Japan's Labor Standards Act, prenatal leave can be requested from 6 weeks before the due date, counting the due date (14 weeks for multiple pregnancies). Postnatal leave is 8 weeks from the day after birth; after 6 weeks you may work if you request it and a doctor approves. The dates shown assume the baby is born exactly on the due date.
In Japan, children born from April 2 to April 1 of the following year are in the same school year (because, legally, a person ages a year on the day before their birthday). Children born from January 1 to April 1 are called "early-born" (hayaumare).
Results are a guide, not a diagnosis. Follow your clinic's instructions about your health and the timing of birth.
Everything you enter is calculated in your browser and is never sent to a server.

Questions this tool answers

Use Due Date Calculator & Pregnancy Weeks (What Week Am I Today?) for searches like these:

I want to calculate my due date from my last period
I want to know what week and day of pregnancy I am in today
I want to know how many months pregnant I am
I want a pregnancy week chart or calendar
I want to know my due date with a long menstrual cycle
I want to calculate my due date from my ovulation date
I want to calculate my due date from a blastocyst transfer date
I want to know my current week counting back from the due date
I want to calculate a due date with Naegele's rule
I want to know when the final month and full term start
I want to know when the second trimester starts
I want to know the date I reach 36 weeks
I want to check what week of pregnancy a specific date is
I want to calculate when I can start maternity leave
I want to know whether my due date means an early-born school year

FAQ

How do I calculate a due date?: Add 280 days (40 weeks) to the first day of your last period. If your last period started on January 1, 2026, your due date is October 8, 2026. That assumes a 28-day cycle; with a 35-day cycle the due date is 7 days later, October 15.
What is Naegele's rule?: It is a mental-math method: take the first day of your last period, subtract 3 from the month (or add 9 if you cannot subtract), and add 7 to the day. A last period starting January 1 gives October 8. Because it ignores differences in month length, it can differ by a few days from counting 280 days. For example, with a last period on May 31, 2026, the rule gives March 10, 2027, while counting 280 days gives March 7. This tool shows both.
How do I find out what week and day of pregnancy I am in today?: Treat the first day of your last period as 0 weeks 0 days, and divide the number of days up to today by 7: the quotient is the weeks and the remainder is the days. With a last period on June 1, 2026, September 23, 2026 is day 114 (16 weeks 2 days), so you are 16 weeks 2 days pregnant (month 5, second trimester). This tool shows today's week as soon as you pick a date.
Which weeks make up each month of pregnancy?: Each month is counted as 4 weeks. Month 1 is weeks 0-3, month 2 is weeks 4-7, month 3 is weeks 8-11, month 4 is weeks 12-15, month 5 is weeks 16-19, month 6 is weeks 20-23, month 7 is weeks 24-27, month 8 is weeks 28-31, month 9 is weeks 32-35, and month 10 (the final month) is weeks 36-39. Week 40 is the due date.
Does the due date change if my cycle is not 28 days?: Yes. The 280-day calculation assumes a 28-day cycle (ovulation on day 14 after the last period), so people with longer cycles ovulate later and the due date shifts later too: 7 days later for a 35-day cycle, 2 days later for a 30-day cycle. If your cycle is irregular the error can be large, so give priority to the due date set by an early-pregnancy ultrasound.
Can I calculate a due date from an ovulation date or an IVF transfer date?: Yes. The ovulation (conception) date is 2 weeks 0 days, so add 266 days to it to get the due date. For embryo transfers it depends on the embryo's age: add 261 days to the transfer date for a blastocyst (day-5 embryo) or 263 days for an early embryo (day-3). A blastocyst transferred on January 20, 2026 gives a due date of October 8, 2026.
When do full term, the final month, and the second trimester start?: The second trimester (often called the stable period in Japan) starts at 16w0d, the final month (month 10) at 36w0d, and full term runs from 37w0d to 41w6d. With a last period on June 1, 2026 (due date March 8, 2027), the second trimester starts September 21, 2026, the final month on February 8, 2027, and full term runs February 15 to March 21, 2027.
What are preterm and post-term births in terms of weeks?: Births from 22w0d to 36w6d are preterm, 37w0d to 41w6d are full term, and 42w0d onward are post-term. The "Date ⇔ week" tab shows which category a birth on the selected date would fall into, plus the dates of each category for this due date.
Can I calculate when I will reach week 36?: Yes. On the "Date ⇔ week" tab, enter "36" or "36w2d" under "When do I reach pregnancy week...?" and the date appears. With a last period on June 1, 2026, 36w0d is February 8, 2027. Conversely, pick the date of an appointment or a trip and it shows which week and day that is.
When can I start maternity (prenatal) leave?: You can start 6 weeks before the due date, counting the due date (14 weeks before for twins or more). With a due date of October 1, 2026, prenatal leave starts August 21, 2026 (June 26 for multiples). Postnatal leave runs 8 weeks from the day after birth, so for a birth on the due date it ends November 26.
My due date is in early April. Will my child be early-born?: A child born by April 1 is early-born and enters the earlier school year; a child born on April 2 or later is in the next year. For example, someone born April 1, 2027 enters elementary school in April 2033, and someone born April 2 enters in April 2034. If the April 1/2 boundary falls within two weeks either side of your due date, the tool shows a notice.
My due date differs from what my clinic told me.: In early pregnancy (around weeks 8-11) an ultrasound measures the baby's crown-rump length (CRL), and if it differs from the date based on your last period by about a week or more, the due date is usually corrected to the ultrasound result. Once your clinic sets a due date, choose "From due date" so the weeks and calendar match your clinic's counting.

Special Thanks

SOLO GUILD | サービス作るAIエンジニア

Back to all tools