Your Pregnancy Week by Week: A Trimester-by-Trimester Overview

Pregnancy is 40 weeks long, divided into three trimesters that each bring distinct physical changes, developmental milestones, and emotional experiences. No two pregnancies are identical, but the broad arc of what happens week by week follows a well-understood pattern — and knowing what to expect at each stage makes the whole experience considerably less bewildering.

This guide is a reference overview of the full pregnancy from conception to birth. Each trimester is covered in detail across its key weeks, with the most significant developments and what they mean for you practically. Use it as a starting point and follow the links throughout to the more detailed posts on each topic.

How pregnancy is counted

Pregnancy is measured from the first day of your last menstrual period rather than from the date of conception. This means that by the time a positive pregnancy test appears — typically around week four — you are already considered four weeks pregnant even though fertilisation occurred approximately two weeks earlier.

A full-term pregnancy is 40 weeks, though births between 37 and 42 weeks are considered within the normal range. Your estimated due date is calculated as 280 days from the first day of your last period, and your dating scan between 10 and 13 weeks will confirm or adjust this date based on your baby's measurements.

First trimester: weeks 1 to 12

The first trimester is the period of most rapid and foundational development. Your baby grows from a single fertilised cell to a fully formed fetus with a beating heart, recognisable facial features, and all major organ systems in place — in just twelve weeks. Meanwhile your body is doing more hormonal and physiological work than at almost any other point in the pregnancy, mostly invisibly.

Weeks 1 to 4

Fertilisation occurs in week two. The fertilised egg travels to the uterus and implants in the uterine wall around week three, triggering the production of human chorionic gonadotropin — the hormone detected by a pregnancy test. By week four, hCG levels are high enough to produce a positive test result in most women. Implantation sometimes causes light spotting, which is normal and often mistaken for a light period.

Your body in these early weeks is flooding with progesterone, which is responsible for the fatigue and breast tenderness that many women notice before they even know they are pregnant. The neural tube — the structure that will become your baby's brain and spinal cord — begins forming in week three. This is why folate supplementation before conception and in the earliest weeks is so critical.

Weeks 5 to 8

This is typically when pregnancy symptoms arrive in force. Nausea — which can occur at any time of day despite its name — affects the majority of pregnant women and is driven by rising hCG levels. Fatigue deepens. Breast tenderness becomes more pronounced. Frequent urination begins as blood flow to the kidneys increases.

Your baby develops rapidly in these weeks. A heartbeat is detectable by ultrasound from around week six. Arm and leg buds appear. The face begins to take shape, with eye and ear structures forming. By week eight your baby is around 1.6 centimetres long and is referred to as a fetus rather than an embryo.

Nausea management in this period typically involves small frequent meals, avoiding triggers, staying hydrated, and for some women ginger-based foods or products. If nausea prevents you from keeping any fluid down for more than 24 hours, contact your GP — this may indicate hyperemesis gravidarum, which requires medical treatment.

Weeks 9 to 12

By week 12 your baby has fully formed fingers and toes, functional kidneys, and a liver that has begun producing blood cells. All major organ systems are in place, though most continue developing throughout the pregnancy. Your baby is around 5.4 centimetres long and weighs approximately 14 grams.

Your first antenatal appointment — the booking appointment — typically occurs in this window. It covers your full medical history, blood and urine tests, blood pressure, and a discussion of your care options. Your dating scan and first trimester screening, which assesses the risk of chromosomal conditions, are offered between weeks 10 and 13.

For most women, nausea begins to ease toward the end of the first trimester, though for some it continues into the second trimester. The risk of miscarriage drops significantly after a heartbeat is confirmed and further after the dating scan — which is why many people choose to share their pregnancy news after this appointment.

For a more detailed guide to this period, read our first trimester guide: what to expect.

Second trimester: weeks 13 to 27

The second trimester is widely considered the most manageable period of pregnancy. The acute hormonal symptoms of the first trimester ease for most women, energy returns, and the pregnancy becomes visible to the world. It is also the trimester of some of the most significant fetal development and the first time most women feel their baby move.

Weeks 13 to 16

Your uterus moves out of the pelvis and into the abdominal cavity, which is why the bump begins to show externally around this time — earlier for women who have been pregnant before. The round ligaments supporting the uterus stretch to accommodate this growth, producing the quick, sharp pains in the lower abdomen that are one of the most common second trimester complaints.

Your baby's facial muscles are developing sufficiently to produce expressions. Fingerprints form. Fine hair called lanugo covers the body. By week 16 your baby is around 11 centimetres long.

Weeks 17 to 20

This is typically when fetal movement is first felt — described most often as a flutter, bubble, or gentle tapping. For first-time mothers, this usually happens between weeks 18 and 22. For women who have been pregnant before, recognition of movement often comes earlier because they know what to look for.

The anatomy scan — also called the morphology or mid-pregnancy ultrasound — is offered between weeks 18 and 20. This detailed scan checks your baby's physical development across more than 20 structural measurements including the brain, heart, spine, kidneys, and limbs. It also checks placental position and amniotic fluid volume, and can reveal your baby's sex if you choose to find out.

Weeks 21 to 27

Your baby grows from around 26 centimetres at week 21 to around 36 centimetres at week 27, gaining significant weight and beginning to develop the fat stores that will regulate their temperature after birth. The lungs begin producing surfactant — the substance that allows them to inflate after birth — from around week 24.

From your perspective, the second trimester in these later weeks brings increasing back and hip discomfort as your posture adjusts to the growing bump and relaxin continues to loosen your pelvic ligaments. Heartburn begins for many women as the uterus puts pressure on the stomach. Leg cramps are common, particularly overnight.

Sleep starts to become more challenging in the final weeks of the second trimester. Side sleeping becomes the recommended position and the bump begins to require support overnight. This is the window — roughly 18 to 22 weeks — when most women benefit most from starting to use a pregnancy pillow, before discomfort becomes significant and before the third trimester demands arrive.

For more on this trimester, read our second trimester guide: what changes and what to expect.

Third trimester: weeks 28 to 40

The third trimester begins at week 28 with a significant shift in the practical and medical management of pregnancy. NSW Health's recommendation to settle to sleep on your side from 28 weeks applies from this point. Appointments become more frequent. Your baby gains the majority of their birth weight. And the physical demands on your body reach their peak.

Weeks 28 to 32

Your baby's brain develops rapidly in these weeks, forming the folds and grooves that characterise the mature brain. The eyes open and close. Rhythmic breathing movements begin as practice for after birth. By week 32 your baby is around 42 centimetres long and weighs approximately 1.7 kilograms.

From 28 weeks you should be aware of your baby's movement pattern and notice if it changes. There is no defined number of movements that is normal — your baby's individual pattern is what matters. If you notice a significant reduction, contact your midwife promptly. The Safer Baby Bundle at stillbirthcre.org.au provides clear guidance on this.

Key medical events in this period include the glucose tolerance test if not already completed, a blood test checking iron levels and other routine markers, and the whooping cough vaccination recommended between 28 and 32 weeks to protect your newborn before they can be vaccinated themselves.

Weeks 33 to 36

Your baby is gaining around 200 grams per week in this period as fat stores accumulate. The lanugo that covered the body begins to shed. By week 36 your baby is around 47 centimetres and most are in a head-down position in preparation for birth.

At around 35 to 37 weeks, a Group B Streptococcus swab may be offered to check for a bacteria that some women carry without symptoms but that can cause serious infection in newborns if transmitted during birth. A positive result means antibiotics are offered during labour.

Sleep in the third trimester is genuinely challenging for most women. The combination of a large bump, hip and pelvic pain from side lying, frequent bathroom trips, heartburn, and Braxton Hicks creates the fragmented sleep pattern that defines this period. Getting your sleep setup right — with support under your bump, between your knees, and behind your lower back — makes a measurable difference. The Bumpnest Maternity Pillow is designed specifically for this stage.

Weeks 37 to 40

Pregnancy is considered full term from 37 weeks. Your baby is gaining weight and putting on the final layers of fat that will help regulate temperature after birth. The lungs are mature. The brain continues developing — importantly, it continues developing for years after birth.

Your baby may engage — move lower into the pelvis — in these final weeks, which for many women brings a noticeable shift: more room to breathe, but increased pelvic pressure and more frequent urination. This lightening is more common in first pregnancies.

Antenatal appointments become weekly or every two weeks from 36 weeks. Your care team will check your baby's position and discuss your birth preferences in detail. If you haven't already, pack your hospital bag — ideally by 36 weeks — and complete your antenatal education.

Braxton Hicks become more frequent and sometimes more noticeable in the final weeks. True labour contractions differ by becoming progressively stronger, longer, and closer together. When contractions are every three to five minutes, lasting around one minute each, for at least an hour, it is generally time to contact your care team or head to hospital. 

For a comprehensive guide to this period, read our third trimester guide: what to expect in the final stretch.

Your due date and what comes after

Your due date is an estimate. Only around five percent of babies are born on their due date. Most births occur in the window between 39 and 41 weeks, and pregnancies are typically managed until 41 to 42 weeks before induction is discussed.

Going past your due date is common and does not indicate that something is wrong. Your care team will increase monitoring in the final days and weeks and discuss induction timing based on your individual circumstances.

After birth, the fourth trimester begins — the twelve weeks of physical recovery, hormonal adjustment, and newborn care that is as significant as any of the three trimesters before it. For an honest account of what that period looks like, read our [fourth trimester guide: what nobody warns you about].

Key appointments across the full pregnancy

First trimester: Booking appointment at 8 to 12 weeks. Dating scan and first trimester screening at 10 to 13 weeks.

Second trimester: Routine antenatal appointments every four weeks. Anatomy scan at 18 to 20 weeks. Glucose tolerance test at 24 to 28 weeks if indicated.

Third trimester: Appointments every two to four weeks from 28 weeks, increasing to every one to two weeks from 36 weeks. Glucose tolerance test at 26 to 28 weeks. Whooping cough vaccination at 28 to 32 weeks. Group B Streptococcus swab at 35 to 37 weeks. Birth preferences discussion and hospital registration in the final weeks.