How to Calculate Pregnancy Weeks and Your Due Date

Learning how to calculate pregnancy weeks helps you follow your pregnancy with confidence, because almost every checkup, test and milestone is scheduled by week rather than by month. Pregnancy is counted from the first day of your last menstrual period, known as the LMP, even though conception usually happens about two weeks later. That convention gives a standard length of 40 weeks, or 280 days, from the LMP to the due date. The method works out the due date, the current week and the trimester from that LMP date, and the tool presents its results in Tamil for Tamil readers, while this guide explains the same method in English so you can check any result by hand. To check your own dates, use the free Pregnancy Calculator in Tamil.

Counting from the LMP can feel strange at first, because it means the first two weeks of the counted pregnancy happen before conception. Doctors still use it because the LMP is usually a date a woman remembers, while the exact day of conception is rarely known. Once you have the LMP, the rest is simple arithmetic: add 280 days for the due date, and count the days since the LMP to find the current week. This guide walks through that method step by step, works the tool page’s example in full, and answers the questions readers ask most often.

The Method / Formula

The method rests on Naegele’s rule, the standard rule for estimating a due date from the last menstrual period. It assumes a regular 28-day cycle and a pregnancy lasting 40 weeks. Two quantities come out of it: the estimated due date, and the gestational age, which is how far along the pregnancy is on any given day.

Due date = LMP + 280 days (40 weeks). Gestational age = the number of days from the LMP to the chosen date, expressed as completed weeks plus extra days. Trimesters split the 40 weeks into three parts: the first trimester covers weeks 0 to 13, the second covers weeks 14 to 27, and the third runs from week 28 until delivery.

LMP: the first day of the last menstrual period, the starting point for all counting. Due date: the estimated delivery date, 280 days after the LMP; it is an estimate, and only about 5% of babies arrive exactly on it. Gestational age: the age of the pregnancy, written as weeks and days, for example 10 weeks 0 days. Trimester: which third of the pregnancy the current week falls in. As-on date: the date for which the age is calculated, usually today.

How to Calculate Pregnancy Weeks Step by Step

1. Write down the first day of your last menstrual period (LMP).
2. Add 280 days (40 weeks) to the LMP to get the estimated due date.
3. Count the days from the LMP to the date you want to check, usually today.
4. Divide that day count by 7: the whole number is the completed weeks, and the remainder is the extra days.
5. Match the week to its trimester: weeks 0 to 13 are the first, weeks 14 to 27 the second, and week 28 onwards the third.
6. Enter the same LMP into the calculator to check your result, remembering the tool shows its results in Tamil.

Worked Example

Take the tool page’s example: an LMP of 2026-01-01, checked as on 2026-03-12. Working through the method gives:

• Due date = 2026-01-01 + 280 days = 2026-10-08.
• Days from LMP to 2026-03-12 = 70 days, which is 10 weeks 0 days pregnant.
• Week 10 falls in weeks 0 to 13, so the trimester is Trimester 1 — exactly as the tool page shows.

A due date of 2026-10-08 is an estimate, not an appointment. Only about 5% of babies are born on their due date, and the date is most useful as a planning point for checkups and leave. If the LMP is unknown or cycles are irregular, a dating ultrasound gives a more accurate estimate than counting from an uncertain LMP.

Common Mistakes / Good to Know

• Counting from conception: pregnancy weeks are counted from the LMP, not from conception, so counting from conception puts you about two weeks behind.
• Treating the due date as exact: only about 5% of deliveries happen on the due date, so treat it as an estimate, not a fixed day.
• Using an uncertain LMP: if the LMP is unknown or cycles are irregular, the calculation is shaky — a dating ultrasound is more accurate.
• Mixing up weeks and months: pregnancy is tracked in weeks, and months do not divide evenly into weeks, so a ‘month’ count can mislead.

For a different way to track the same dates, try the OB Wheel Calculator, and read our related guide, How to Calculate Your Due Date from LMP.

Frequently Asked Questions