First Trimester: What to Expect

What Is the First Trimester?

The first trimester is the first of pregnancy's three stages, running from week 1 to the end of week 13. It is the opening stage of the pregnancy trimesters, the three-stage framework that divides a full-term pregnancy into equal stretches. The first trimester begins pregnancy and sets the baseline the later stages build on. This page explains when the first trimester starts, how many weeks it lasts, what to expect during it, and which nutrients matter most at this stage.

When Does the First Trimester Start?

The first trimester starts on the first day of the last menstrual period, which is counted as week 1. This dating follows the Australian Government Department of Health's Clinical Practice Guidelines: Pregnancy Care. Because the count begins from the last menstrual period, week 1 falls about two weeks before conception.

How Many Weeks Is the First Trimester?

The first trimester lasts about 13 weeks, covering weeks 1 to 13 of pregnancy. It covers the first third of the roughly 40-week pregnancy under the same clinical dating convention. The stage ends at the close of week 13, when the second trimester begins.

What Are Common First Trimester Symptoms?

Common first trimester symptoms are the typical physical changes many women notice in early pregnancy. Recognised in Australian pregnancy care guidance, the common first trimester symptoms are listed below:

• Nausea: queasiness or morning sickness, often occurring at any time of day.
• Tiredness: increased tiredness as the body adjusts to early pregnancy.
• Breast tenderness: sensitivity of the breasts.
• Food aversions: a reduced appetite for foods eaten comfortably before pregnancy.

These symptoms are typical of the first trimester and often ease as pregnancy moves into the second trimester.

Which Nutrients Matter Most in the First Trimester?

Folate is the priority nutrient in the first trimester of pregnancy. The NHMRC Nutrient Reference Values for Australia and New Zealand emphasise folate intake as a focus in the early weeks of pregnancy. For how much folate to aim for and how to meet the need, see the guide to folate in pregnancy.

How Does First-Trimester Nutrition Fit Your Whole Pregnancy?

First-trimester nutrition is the starting point of a shifting, whole-pregnancy nutrition picture. Nutrient needs change from stage to stage, so the folate focus of early pregnancy gives way to different priorities in the second and third trimesters. For a more complete view of women's nutrition at this life stage, including how these pregnancy dietary needs develop across every trimester, see the whole-pregnancy guide to pregnancy dietary needs.