The Three Trimesters of Pregnancy: A Simple Overview
Updated September 2025 · 7 min read
Pregnancy is divided into three trimesters: the first (weeks 1–12), when the baby’s major organs form and early symptoms like nausea are common; the second (weeks 13–27), often the most comfortable, when many parents feel the baby move and can learn the sex; and the third (weeks 28–40+), when the baby grows rapidly and gets ready to be born. A full-term pregnancy is around 40 weeks.
- First trimester: weeks 1–12 — organs form; early symptoms common.
- Second trimester: weeks 13–27 — often easier; movement and anatomy scan.
- Third trimester: weeks 28–40+ — rapid growth and preparing for birth.
- Attend your antenatal appointments and raise any concerns with your care team.
How pregnancy is measured
Pregnancy is counted in weeks from the first day of your last period, and lasts around 40 weeks in total. Those weeks are grouped into three trimesters, each with its own general character. Every pregnancy is different, so treat the following as a broad map, not a rulebook.
First trimester (weeks 1–12)
A huge amount happens early. The baby develops from a cluster of cells into a tiny form with a beating heart and the beginnings of major organs. For the pregnant parent, this is often when early signs appear — tiredness, tender breasts, nausea (“morning sickness,” which can strike any time of day) and needing to wee more. It’s usually when you’ll have your first antenatal (booking) appointment and, often, an early scan.
Second trimester (weeks 13–27)
Many people find this the most comfortable stretch — early nausea often eases and energy returns. Key milestones cluster here: the mid-pregnancy anatomy scan (usually around 18–22 weeks), often the chance to find out the baby’s sex, and, for many, the first flutters of the baby moving. The bump becomes visible.
Third trimester (weeks 28–40+)
The baby grows quickly, gains weight and settles into position for birth. The pregnant parent may feel more tired and uncomfortable again, with more frequent appointments to check on baby’s growth and wellbeing. This is the time to prepare — a birth plan, hospital bag, car seat and the nursery.
Looking after yourself and getting care
Throughout pregnancy, your midwife or doctor will guide your care and monitoring. Attend your appointments, take any recommended supplements such as folic acid, and — importantly — contact your care team promptly about any symptoms that worry you, rather than waiting for the next appointment.
Frequently asked questions
How many weeks are in each trimester?
The first trimester is weeks 1u201312, the second is weeks 13u201327, and the third is weeks 28 until birth, usually around 40 weeks in total.
Which trimester is the hardest?
It varies. Many find the first trimester tough due to nausea and tiredness, the second the most comfortable, and the third physically demanding as the baby grows.
How long is a full-term pregnancy?
A full-term pregnancy is around 40 weeks, counted from the first day of your last period, though babies are considered full term from 37 weeks.
Sources
This article is general information, not medical advice. Pregnancy care is personal — always follow the guidance of your own midwife, obstetrician or GP, and contact them promptly about any symptoms or concerns. Guidance reflects widely published advice from bodies such as the NHS and the American College of Obstetricians and Gynecologists (ACOG).