Pregnancy weeks to months: conversion table and trimesters

Doctors count in weeks, so months are an approximation

ACOG says a normal pregnancy lasts about 40 weeks from the first day of the last menstrual period, divides it into three trimesters, and describes a trimester as a 3-month period in its glossary (ACOG, "How Your Fetus Grows During Pregnancy", 2024). The NHS week-by-week guide (NHS, 2023) and the Office on Women's Health page "Stages of pregnancy" (2025) also count in weeks. None of the three pages gives a weeks-to-months table. It is a conversion patients ask for, and the answer depends on what a month is taken to be.

The arithmetic

This table is our own arithmetic, not a medical convention. It divides the days in the number of weeks by 30.44, the average length of a calendar month (365.25 days divided by 12), and says which month you are in. Gestational age counts from the first day of the last period (ACOG, 2024).

A pregnancy that is 29 weeks along is therefore about 6.7 months, in the 7th month. A full 40 weeks is 9.2 months, which is why a pregnancy is often said to last nine months although the last weeks fall in a tenth. The rule of thumb of 4 weeks to a month gives a different answer: it makes 29 weeks 7.25 months and 40 weeks 10 months, because a calendar month is slightly longer than 4 weeks. Neither answer is wrong, and none of it changes how a clinician counts. The pregnancy week calculator on this site gives your gestational age in weeks and days and the trimester.

Months and pregnancy trimesters

ACOG, the NHS and the US Office on Women's Health draw the trimester lines in different weeks: ACOG ends the first trimester at 13 weeks 6 days, the NHS week-by-week guide groups weeks 4 to 12 as the first trimester, and the Office on Women's Health uses weeks 1 to 12 (ACOG, "How Your Fetus Grows During Pregnancy", 2024; NHS, 2023; Office on Women's Health, 2025). The hub for pregnancy trimesters sets all three trimesters side by side, the first trimester guide places that trimester's tests and appointments against these weeks, and estimated gestational age explains how the weeks are counted.

Why weeks matter more than months

ACOG says the due date determines gestational age throughout pregnancy so that fetal growth can be tracked, and gives a timeline for certain tests (ACOG, 2024). ACOG's opinion on estimating the due date measures scan thresholds in days (ACOG Committee Opinion 700, 2017). A month is too coarse a unit for either purpose, which is why appointments, scans and screening windows are all given in weeks.

This guide is information, not a diagnosis or a recommendation of treatment, and it is no substitute for your own clinician. See the medical disclaimer.