Trimesters of Pregnancy: Stages Explained
- Pregnancy trimesters are the three stages a full-term pregnancy is divided into: the first trimester, the second trimester, and the third trimester.
- Each trimester of pregnancy lasts roughly 13 to 14 weeks, and a full-term pregnancy runs about 40 weeks in total.
- Each trimester covers about three months, so the three trimesters together span roughly nine months.
- Nutrient needs change as pregnancy progresses through its three trimesters.
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Pregnancy trimesters are the three stages a full-term pregnancy is divided into: the first trimester, the second trimester, and the third trimester. Each stage marks a distinct phase of pregnancy, measured in weeks from the start of pregnancy.
The three trimesters follow a fixed order, with the first trimester covering early development, the second the middle of pregnancy, and the third the run up to birth. This page explains how the three trimesters divide pregnancy, how many weeks and months each one covers, when each one starts and ends, what happens in each, and how nutrient needs shift from stage to stage.
What Are the Trimesters of Pregnancy?
Pregnancy trimesters are the three stages a full-term pregnancy is divided into: the first trimester, the second trimester, and the third trimester. Each stage marks a distinct phase of pregnancy, measured in weeks from the start of pregnancy. The three trimesters follow a fixed order, with the first trimester covering early development, the second the middle of pregnancy, and the third the run up to birth. This page explains how the three trimesters divide pregnancy, how many weeks and months each one covers, when each one starts and ends, what happens in each, and how nutrient needs shift from stage to stage.
How Many Weeks Are in a Trimester?
Each trimester of pregnancy lasts roughly 13 to 14 weeks, according to the Australian Government Department of Health's Clinical Practice Guidelines: Pregnancy Care. A full-term pregnancy runs about 40 weeks in total, and the three trimesters divide that span into three near-equal stages. The first and third trimesters each cover about 13 weeks, and the second trimester covers about 14 weeks.
How Many Months Is Each Trimester?
Each trimester covers about three months, so the three trimesters together span roughly nine months. Measured in weeks, pregnancy runs about 40 weeks under the same clinical dating convention, which is a little longer than nine calendar months. The month figure is an approximation, because the weeks of pregnancy do not divide evenly into calendar months.
How Many Weeks Is a Full-Term Pregnancy?
A full-term pregnancy lasts about 40 weeks, counted from the first day of the last menstrual period. The Australian pregnancy care guidelines use this 40-week span as the total that the three trimesters divide. Because the count starts from the last menstrual period rather than conception, the 40 weeks include roughly two weeks before pregnancy begins.
When Does Each Trimester Start and End?
Each trimester of pregnancy starts and ends at a set gestational week, counted from the first day of the last menstrual period. Under the Australian pregnancy care guidelines, the first trimester runs from week 1 to week 13, the second from week 14 to week 27, and the third from week 28 until birth. Week-boundary conventions vary slightly between sources; this page follows the week 1 to 13, 14 to 27, and 28 to birth convention. The start and end week of each trimester are listed below:
| Trimester | Start | End |
|---|---|---|
| First trimester | Week 1 | Week 13 |
| Second trimester | Week 14 | Week 27 |
| Third trimester | Week 28 | Birth, around week 40 |
What Happens in Each Trimester?
Each trimester of pregnancy has its own defining features, from early development through to preparation for birth. The overview below summarises what happens across the three stages, recognised in Australian pregnancy care guidance; each stage is covered in full on its own page. The main features of the three trimesters are set out in the table below:
| Trimester | Defining feature |
|---|---|
| First trimester | Early development, with common symptoms such as nausea and tiredness |
| Second trimester | Early symptoms often ease, and the mid-pregnancy anatomy scan and first movements occur |
| Third trimester | Rapid fetal growth and preparation for birth |
Experiences and timing vary between pregnancies
The three trimesters together tell the sequence of a single pregnancy, with each stage building on the one before it.
How Do Nutrient Needs Change Across the Trimesters?
Nutrient needs change as pregnancy progresses through its three trimesters. Folate, iodine, and vitamin D matter from conception right through to birth. Under the NHMRC Nutrient Reference Values for Australia and New Zealand, folate is emphasised as important in preconception and early pregnancy. Iron requirements are higher during pregnancy than for non-pregnant adults. The NHMRC includes a pregnancy reference value for long-chain omega-3 fats, including DHA. This shift is why nutritional needs at one life stage of pregnancy do not match those at another. This page maps the timeline; it does not set nutrient doses, which belong to each nutrient's own guide. Individual requirements vary. Speak with your health professional about your diet and whether supplementation is appropriate. For a more complete picture of women's nutrition at this life stage, including how the right nutrients and nutrient support are met across all three stages, see the full guide to nutrition during pregnancy.
References and official guidance
- Australian Government Department of Health — Clinical Practice Guidelines: Pregnancy Care
- NHMRC — Nutrient Reference Values for Australia and New Zealand
Experiences and timing vary between pregnancies. Individual requirements vary. Speak with your health professional about your diet and whether supplementation is appropriate.
