Pregnancy Weeks and Calendar Weeks

Two kinds of weeks

People who are expecting a baby meet two kinds of weeks. Pregnancy weeks count from the first day of the last menstrual period and give the gestational age. Calendar weeks, such as ISO week 43, are fixed blocks from Monday to Sunday in the year. They have nothing in common except the length of seven days.

A pregnancy week starts on whichever weekday the count began. If the last period started on a Sunday, every pregnancy week starts on a Sunday. An ISO week always starts on a Monday. So a pregnancy week is almost never the same set of days as a calendar week. This article is general information. Your doctor or midwife decides the dates that apply to you.

How the 40 weeks are counted

A full-term pregnancy is described as 40 weeks, which is 280 days, counted from the first day of the last menstrual period. Conception usually happens about two weeks after that day, so the baby is about 38 weeks old at term. The convention of counting from the last period exists because that date is usually known, while the day of conception is not.

Gestational age is given in completed weeks and days. "33 weeks and 4 days" means that 33 full weeks and 4 more days have passed. A person who is "in week 34" is in the 34th week, which means 33 weeks are completed. It is worth asking which convention is used when two people compare numbers.

Finding the due date

A simple rule for the estimated due date is Naegele's rule: take the first day of the last period, add one year, subtract three months and add seven days. For a last period that began on Monday 5 January 2026, the date is 12 October 2026, which is exactly 280 days later. The rule assumes a cycle of about 28 days, so doctors often adjust the date with an ultrasound.

In calendar-week terms, 12 October 2026 is a Monday in ISO week 42. Only a small share of babies are born on the due date itself, and normal births happen across several weeks. The due date is a middle point of a range, not an appointment.

Trimesters in weeks

Pregnancy is divided into three trimesters. The first trimester runs to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 to birth. A baby is called early term from 37 weeks, full term from 39 weeks and late term from 41 weeks, according to the definitions used by obstetric organisations. The exact limits may vary a little by country.

To place a trimester on the calendar, add the weeks to the last period. With the last period starting on Sunday 1 March 2026, the first trimester ends on Saturday 6 June, the second begins on Sunday 7 June, and the third begins on Sunday 13 September 2026. You can check each date by adding days with the date calculator.

From pregnancy week to calendar week

Suppose the last period began on Sunday 1 March 2026. The due date is Sunday 6 December 2026, which is in ISO week 49. On Thursday 22 October 2026, the day count from 1 March is 235 days, which is 33 weeks and 4 days, while the calendar says week 43 of the year. The difference between 33 and 43 is nothing but a coincidence of the start date.

If you want to plan leave or appointments by calendar week, use the due date to find the ISO week and count backwards. Many employers and insurers in countries that use week numbers talk about the "calendar week of the due date". Always confirm the exact date with the authority or employer, since rules for leave start dates depend on the country.

Why the date can change

The due date from the last period is an estimate, and it can move. If a person's cycle is longer or shorter than 28 days, or if the date of the last period is uncertain, the estimate may be off. An early ultrasound measures the size of the embryo or fetus and is usually used to confirm or correct the date, particularly when the two differ by more than a few days.

When the date changes, every week of the pregnancy shifts with it, and so does the calendar week of the due date. If you have written the ISO week of the due date on a form or in a calendar, update it. The date calculator on this site makes this quick: enter the new start date, add 280 days and read the week from the first tab.

Using the calculator

Use the first tab to find the ISO week of the due date or of an appointment. Use the second tab to turn a week number from an appointment letter into real dates. Use the date calculator on this site to add 280 days to a date or to count the days between two dates.

None of these tools gives medical advice. They only do date arithmetic. Anything that concerns health, the date of birth or leave rights should be confirmed with a doctor, midwife or the relevant office.