Pregnancy Trimesters Explained Simply

Pregnancy is divided into three periods called trimesters, each roughly 12–14 weeks long. Together they span approximately 40 weeks from the first day of your last menstrual period (LMP) to the expected date of birth. Understanding what happens in each trimester helps you know what to expect, when key appointments take place, and how your body and baby are changing.

Key Milestones at a Glance

First Trimester: Weeks 1–12

Understanding the Dates

Confusingly, "week 1" of pregnancy begins on the first day of your last menstrual period — before conception has occurred. Ovulation and fertilisation typically happen in week 2. This means that by the time most women realise they are pregnant (around week 4–5, when a period is missed), the pregnancy is already several weeks along by medical reckoning.

Key Developmental Milestones

The first trimester is the period of greatest developmental complexity. In just twelve weeks, a fertilised egg develops into a fetus with all major organ systems in place:

• Weeks 1–4: Fertilisation, rapid cell division, and implantation in the uterine lining
• Week 5: The embryo's heart begins to beat; visible on early ultrasound around week 6
• Weeks 6–8: Brain, spinal cord, limb buds, and facial features begin forming
• Weeks 9–10: Fingers and toes are distinct; external genitalia begin to develop; from week 10, the embryo is formally called a fetus
• Weeks 11–12: All major internal organs are in place; the fetus is approximately 5–6 cm long and weighs around 14 g

The risk of miscarriage is highest in the first trimester, particularly before week 10, and decreases substantially after the dating scan confirms a healthy heartbeat.

Common Symptoms in the First Trimester

• Nausea and vomiting: Affects around 70–80% of pregnant women. Despite the name, "morning sickness" can strike at any time of day. It typically peaks around weeks 8–10 and improves for most women by the end of the first trimester, though for some it persists longer. Severe, persistent vomiting (hyperemesis gravidarum) requires medical assessment.
• Fatigue: Often described as overwhelming in the first trimester, driven by rising progesterone levels and the effort of building the placenta
• Breast tenderness and changes: Breasts often become fuller, heavier, and more sensitive; the areolas may darken
• Frequent urination: Begins early and is caused by hormonal changes and increased blood volume filtering through the kidneys
• Heightened sense of smell and food aversions: Common and thought to be hormonally driven
• Mood changes: Normal and expected due to significant hormonal shifts

What Changes for the Mother

The uterus begins growing almost immediately, though it is not yet visible as a bump. Blood volume increases by up to 50% over the course of pregnancy, and this process begins early. The placenta forms and takes over hormone production from the corpus luteum at around 10–12 weeks. Many women feel at their most physically uncomfortable during this trimester, even though the pregnancy is not yet visible.

Antenatal Appointments and Milestones

• Booking appointment (weeks 8–12): Your first formal midwife appointment. Involves a full health history, blood pressure, blood tests (blood group, anaemia, HIV, syphilis, hepatitis B, screening for Down's syndrome), urine test, and discussion of care options.
• Dating scan (weeks 11–14): First ultrasound, which confirms the due date and screens for chromosomal conditions including Down's syndrome (combined with a blood test). Often the first time parents see and hear the baby's heartbeat.

Second Trimester: Weeks 13–27

The second trimester is widely described as the most comfortable period of pregnancy. For most women, nausea subsides, energy returns, and the pregnancy becomes visibly noticeable. The risk of miscarriage drops significantly.

Key Developmental Milestones

• Weeks 13–16: Facial expressions develop; the baby can swallow and urinate; sex may become distinguishable on scan
• Weeks 16–20: Bones begin to harden (ossify); lanugo (fine downy hair) covers the skin; the baby grows from around 14 cm to approximately 25 cm
• Weeks 20–24: The baby can hear sounds and respond to light; vernix (a protective waxy coating) forms; fingerprints develop
• Weeks 24–27: The baby reaches viability — with intensive neonatal care, survival is possible if born at this stage, though outcomes improve significantly with each week closer to term

Common Symptoms in the Second Trimester

• Visible and growing baby bump — most noticeable from around 16–20 weeks
• Reduced nausea and improved appetite for many women
• More energy compared to the first trimester
• Back pain and round ligament pain (brief sharp pains on the sides of the abdomen as the uterus stretches)
• Skin changes: stretch marks may appear; a dark line (linea nigra) often develops down the abdomen; some women notice changes in skin pigmentation
• Nasal congestion and occasional nosebleeds (due to increased blood flow to mucous membranes)
• Leg cramps, particularly at night

Feeling the Baby Move

First movements — called "quickening" — are typically felt between 16 and 24 weeks. First-time mothers often notice them later in this range. The sensation is often described as fluttering, bubbles, or gentle tapping. By around 24 weeks, regular movement patterns should be established and you should contact your midwife if you notice a significant reduction.

What Changes for the Mother

The uterus grows from roughly the size of a grapefruit at 13 weeks to a size that reaches the belly button by around 20 weeks. The centre of gravity shifts, often causing changes in posture and contributing to back pain. Blood pressure tends to be at its lowest point in the second trimester. Appetite generally increases as nausea resolves.

Antenatal Appointments and Milestones

• Anomaly scan (weeks 18–21): A detailed ultrasound examining the baby's physical structure — brain, heart, spine, kidneys, limbs, facial features, and placenta position. It can identify findings requiring further monitoring or specialist referral.
• Routine midwife appointments: Blood pressure, urine, and fundal height measurement (measuring the uterus to track growth) at regular intervals

Third Trimester: Weeks 28 to Birth

The third trimester is the final stretch. The baby's major developmental work is largely complete — this period is primarily about growth, maturation, and preparation for life outside the womb.

Key Developmental Milestones

• Weeks 28–32: Brain development accelerates; eyes open and close; the baby develops regular sleep-wake cycles
• Weeks 32–36: Rapid weight gain; body fat accumulates to regulate temperature after birth; the baby typically turns head-down (cephalic position) by 36 weeks
• Weeks 36–40: The lungs produce surfactant (essential for breathing independently); the baby "engages" — the head descends into the pelvis in preparation for birth; from 37 weeks the pregnancy is considered full term

Common Symptoms in the Third Trimester

• Significant physical discomfort as the baby grows and fills the abdomen
• Heartburn and acid reflux as the growing uterus pushes on the stomach
• Shortness of breath as the uterus presses on the diaphragm (this often eases when the baby engages)
• Difficulty sleeping due to size, frequent urination, and discomfort
• Braxton Hicks contractions: irregular, painless tightening of the uterus — "practice contractions" that are distinct from true labour
• Swelling in the feet, ankles, and hands (mild swelling is common; sudden severe swelling should be reported immediately)
• Increased frequency of urination as the baby's head presses on the bladder
• Pelvic girdle pain (SPD) affects some women, causing significant discomfort around the hips and pubic bone

What Changes for the Mother

The uterus expands to fill most of the abdominal cavity. The body begins preparing for labour through hormonal changes that soften the cervix and ligaments. Many women experience the "nesting instinct" — a strong urge to organise, clean, and prepare. Colostrum (the early form of breast milk) may begin to be produced.

Antenatal Appointments and Milestones

• Increased appointment frequency: From 28 weeks, midwife appointments occur every 2–4 weeks; from 36 weeks, often every week
• Growth scans: If there are concerns about the baby's growth or placenta position, additional ultrasound scans will be arranged
• Baby position check (36 weeks): If the baby is not head-down (breech), options including external cephalic version (ECV) may be discussed
• Group B Streptococcus testing (if applicable): Not routinely offered on the NHS but available privately; recommended by ACOG in the US at 35–37 weeks
• Antenatal classes: Usually attended in the third trimester, covering labour, birth options, and newborn care
• Birth plan: Discussions with your midwife about your preferences for labour, pain relief, and delivery
• Post-dates monitoring: If labour has not begun by 41 weeks, induction will typically be discussed; most NHS trusts offer induction between 41 and 42 weeks

Preparing for Birth: What to Expect Towards the End

As you approach your due date, signs that labour may be approaching include:

• A "show" — the mucus plug that sealed the cervix during pregnancy may come away, often appearing as a small amount of blood-tinged mucus
• Your waters breaking (rupture of membranes) — contact your maternity unit immediately if this happens
• Regular contractions that become progressively stronger, longer, and closer together

Contact your midwife or maternity unit if you are unsure whether you are in labour, if your waters break, if you experience reduced fetal movement, or if you have heavy bleeding.

Use our Pregnancy Due Date Calculator to calculate your estimated due date and key milestones by trimester.

Related Guides

• Why Due Dates Change During Pregnancy — why your EDD may be revised at scans
• How to Calculate Your Due Date — LMP and conception-based methods
• What Is Gestational Age? — how pregnancy weeks are counted
• LMP vs Conception Date — which dating method is more accurate
• What to Expect in the First Weeks of Pregnancy — early symptoms, booking appointment, and immediate steps

Sources: NHS Pregnancy week by week (nhs.uk); American College of Obstetricians and Gynecologists (ACOG) patient resources; NICE Antenatal Care guidelines (NG201). This guide is for general information only. For questions about your specific pregnancy, always speak to your midwife or doctor.