Pregnancy Trimesters Explained Simply

Pregnancy is divided into three periods called trimesters, each roughly 13 weeks long. Understanding what happens in each one can help you know what to expect and when important milestones tend to occur.

First Trimester: Weeks 1–13

The first trimester begins on the first day of your last menstrual period. At conception (around week 2), the fertilised egg begins dividing rapidly. By week 4, the embryo has implanted in the uterine wall.

What Is Developing

In just the first 12 weeks, the embryo develops from a cluster of cells into a recognisable baby with all major organ systems in place. The brain, heart, lungs, limbs, fingers and facial features all form during this period. By week 10, the baby is called a fetus.

Common Symptoms

• Nausea and vomiting ("morning sickness") — can occur at any time of day, typically peaks around weeks 8–10 and improves by the end of the first trimester for most women
• Fatigue — very common, driven by rising progesterone levels
• Breast tenderness and changes
• Frequent urination
• Heightened sense of smell

Antenatal Milestones

• Booking appointment with your midwife (usually 8–12 weeks)
• First trimester blood tests and screening
• Dating scan (11–14 weeks) — confirms due date and checks for chromosomal conditions

Second Trimester: Weeks 14–26

The second trimester is often called the most comfortable stage of pregnancy. Energy levels frequently improve, nausea typically subsides, and the pregnancy becomes visibly noticeable.

What Is Developing

The baby grows rapidly during this period — from around 9 cm at 14 weeks to about 35 cm by 26 weeks. Bones harden, hair begins to grow, and the baby begins to move. You will likely feel the first movements (called quickening) somewhere between 16 and 24 weeks.

Common Symptoms

• Reduced nausea for many women
• Increased energy
• A visible baby bump
• Back pain as the uterus grows
• Skin changes (stretching, pigmentation)
• Possible round ligament pain (brief sharp pains on the sides of the abdomen)

Antenatal Milestones

• Anomaly scan (18–21 weeks) — a detailed scan checking for structural abnormalities
• Ongoing midwife appointments

Third Trimester: Weeks 27 to Birth

The third trimester is the final stretch. The baby continues to grow significantly, preparing for life outside the womb.

What Is Developing

By 28 weeks, the baby could survive with intensive medical support if born early. By 37 weeks, a pregnancy is considered full term. The baby will typically settle into a head-down position in preparation for birth. The lungs mature in the final weeks.

Common Symptoms

• Increasing size and physical discomfort
• Difficulty sleeping (partly due to size, partly due to frequent urination)
• Braxton Hicks contractions (irregular practice contractions)
• Swelling in the feet and ankles
• Heartburn and shortness of breath as the uterus pushes on the diaphragm
• Nesting instinct

Antenatal Milestones

• Increased frequency of midwife appointments
• Check for the baby's position (usually at 36 weeks)
• Group B Streptococcus (GBS) testing if indicated
• Birth planning discussions
• From 41 weeks: discussions about induction if labour has not started

This guide is for general information only. For questions about your specific pregnancy, always speak to your midwife or doctor.