How to Calculate Your Due Date: 280 Days from Your Last Period, and Naegele's Rule

The basic rule: first day of your last period + 280 days

Your due date is the 280th day (40 weeks 0 days) counting the first day of your last menstrual period as "0 weeks 0 days" of pregnancy. If your last period started on August 3, 2026, the due date is 280 days later, on May 10, 2027.

It may seem odd to count from the start of your last period, when you weren't yet pregnant. The reason is that most people don't know the day they ovulated or conceived, but many remember the day their period started. For someone with a 28-day cycle, ovulation and fertilization happen around day 14 after the last period (2 weeks 0 days), and the baby is born about 266 days after that. That's what the 280 days are made of.

1. 1

   Confirm the first day of your last period

   The day it started, not the day it ended. Use the record in your app or diary if you have one.

2. 2

   Add 280 days

   August 3, 2026 + 280 days = May 10, 2027. On a calendar, it's the same weekday 40 weeks later (a Monday in this example).

3. 3

   Adjust if your cycle isn't 28 days

   For a 35-day cycle, move it 7 days later; for a 25-day cycle, 3 days earlier (more on this below).

How Naegele's rule works, and how it differs from the 280-day count

A method often introduced in Japan's maternal and child health handbook and by maternity clinics for getting a due date without a calendar is Naegele's rule. From the first day of your last period, subtract 3 from the month (or add 9 if you can't) and add 7 to the day, and that's your due date.

• Last period August 3 → month 8 − 3 = 5, day 3 + 7 = 10 → May 10
• Last period January 31 → month 1 + 9 = 10, day 31 + 7 = 38 → October has 31 days, so November 7

It's handy, but because it ignores how many days each month has (28 to 31), it can be a few days off the 280-day count. It's especially likely to drift when February falls in between, or when your last period started at the end of a 31-day month.

The difference is a few days at most, which is fine for a rough idea of timing. But when you need the exact date, for applying for maternity leave or planning to go back to your hometown to give birth, use the 280-day count (or the date set by your clinic, if there is one).

Adjusting when your cycle isn't 28 days

The 280-day calculation assumes ovulation on day 14 after your last period, in other words a 28-day cycle. The time from ovulation to the next period (the luteal phase) is roughly constant at about 14 days for everyone, so people with longer cycles are thought to ovulate later, and people with shorter cycles earlier. So the due date is shifted by however many days your cycle differs from 28 (the modified Naegele's rule).

Shifting the due date also shifts the start of the week count (0 weeks 0 days) by the same number of days. With a 35-day cycle, 0 weeks 0 days is August 10, seven days after the last period started. However, if your cycle varies every time, or you can't estimate ovulation because you're breastfeeding or have just stopped the pill, this adjustment isn't very accurate either. If you know your ovulation date, calculating from your ovulation or embryo transfer date is more accurate.

Why the clinic may revise your due date

A due date calculated from your last period rests on the assumption that you ovulated on day 14 of a 28-day cycle. In reality, ovulation commonly shifts by several days either way. So in early pregnancy (around weeks 8 to 11), clinics measure the baby's length from head to bottom (crown-rump length, CRL) by ultrasound and confirm the due date from that size.

• Babies in early pregnancy vary little in size, so estimating the week from CRL is highly accurate
• If the due date from your last period and the one from CRL differ substantially (roughly a week or more), the due date is often revised to match the CRL result
• Once confirmed, the due date generally isn't changed at later checkups, even if the baby measures large or small (size differences are treated as individual variation)

When the due date is revised, the weeks and calendar shift with it. If the weeks you calculated online don't match your maternal and child health handbook, this revision is usually why. From then on, use the due date set by your clinic as the reference.

Only about 5% of babies arrive on the due date

The due date isn't the day the baby will be born. It's the center of the window when birth is likely. Only around 5% of babies are born on the due date itself. In practice, the five weeks from 37 weeks 0 days to 41 weeks 6 days are called "term", the period when birth is expected.

For traveling to your hometown to give birth or arranging care for older children, it's reassuring to prepare with 37 weeks, when term begins, as the target rather than the due date. Guides to milestones along the way, such as confirming the heartbeat and the stable period, are summarized in Pregnancy schedule by week.

Calculating your due date and today's week together

In the Due Date & Pregnancy Week Calculator, choose "Last period" under "What do you want to calculate from?" and just enter the day your period started. It shows your due date, today's pregnancy week (X weeks Y days) and the pregnancy month. If your cycle isn't 28 days, changing the menstrual cycle field applies the same adjustment as the table above.

Below the result, it also shows the date given by Naegele's rule and how many days it differs from the 280-day due date, which is handy when comparing with the quick-reference chart in your maternal and child health handbook. Once your clinic sets a due date, switch to "From due date" and enter it so the weeks and calendar match the clinic's count.

FAQ

How many days after my last period is the due date?

280 days (40 weeks 0 days) after the first day of your last period. If your last period started on August 3, 2026, your due date is May 10, 2027. This assumes a 28-day cycle. With a 35-day cycle it's 7 days later, May 17, and with a 25-day cycle 3 days earlier, May 7.

The due date from Naegele's rule differs from my app's.

Naegele's rule calculates "month − 3 (or + 9), day + 7" and ignores how many days each month has, so it can be a few days off the 280-day count. For example, with a last period on May 31, 2026, the rule gives March 10, 2027, while counting 280 days gives March 7. When you need the exact date, use the 280-day count.

My periods are irregular and I'm not sure when my last one was. How is my due date set?

In early pregnancy (around weeks 8 to 11), the baby's crown-rump length (CRL) is measured by ultrasound, and the due date is set from that size. If you don't know your last period or your cycle is irregular, this takes priority over the date calculated from your last period. Once the clinic gives you a due date, count weeks from that.

Can a due date change after it's been set?

Once confirmed by early-pregnancy ultrasound, it's generally not changed. Even if later checkups say the baby is large or small, that's treated as individual variation in growth and the due date stays the same.