Your Pregnancy Week by Week: What to Expect

1. First Trimester — Weeks 1 to 12

The first trimester begins on the first day of your last menstrual period (LMP), even though conception typically occurs around week 2. By the time most women discover they are pregnant — usually around weeks 4–6 — significant fetal development is already underway.

Weeks 1–4: Conception and Implantation

During weeks 1–2, your body prepares for ovulation. Conception typically occurs around day 14 of a 28-day cycle. The fertilised egg (zygote) travels through the fallopian tube over 3–5 days, dividing continuously as it goes. By week 4, it implants in the uterine lining and the placenta begins to form. The hormone hCG (human chorionic gonadotropin) — the hormone detected by pregnancy tests — begins to rise.

Weeks 5–8: Rapid Early Development

This is a period of dramatic growth. The embryo goes from the size of an apple seed to around 1.6 cm by the end of week 8. The heart begins beating around week 5–6 (though it is not audible by stethoscope until week 10–12). By week 8, the facial features, limb buds, and the beginnings of all major organs are forming.

Weeks 9–12: Transition to Fetus

At 9 weeks, the embryo is officially called a fetus. By week 12, it measures approximately 5.4 cm and weighs around 14g. Fingers and toes are distinct, the kidneys are producing urine, and the neck has formed. The risk of miscarriage drops significantly after week 10.

2. Second Trimester — Weeks 13 to 27

For many women, the second trimester brings relief from early pregnancy symptoms. Energy levels often improve, and the bump becomes visible. This is also the period when fetal movement — known as quickening — is typically first felt, usually between weeks 16 and 22 for first-time mothers.

Weeks 13–16

The fetus grows from about 7 cm to 12 cm. Fingernails and toenails are forming, the sex organs are developing (though sex may not yet be detectable on ultrasound), and the fetus begins to practise breathing movements using amniotic fluid. Many women have their 16-week midwife appointment during this period.

Weeks 17–22: The Anatomy Scan

The anomaly scan (or anatomy scan) is typically offered between weeks 18 and 21. This detailed ultrasound checks for structural abnormalities in the brain, heart, kidneys, limbs, and face. By week 20, the fetus measures around 25 cm from head to heel — about the length of a banana.

Weeks 23–27: Viability

The fetus reaches "viability" at approximately 22–24 weeks — the earliest point at which it could potentially survive outside the womb with intensive medical care. Survival rates improve significantly week by week from this point. By week 27, the fetus weighs approximately 875g and the lungs are developing the surfactant that will be needed for breathing.

3. Third Trimester — Weeks 28 to 40

The final trimester is characterised by rapid weight gain (both for the mother and baby), increasing discomfort, and preparation for birth. The fetus gains approximately 200g per week during this period.

Weeks 28–32

The brain grows rapidly and the senses develop — the fetus can see light through the uterine wall, hear sounds from outside the womb, and taste amniotic fluid. By week 32, the fetus typically turns head-down in preparation for birth. Most organs are now fully formed.

Weeks 33–36: "Late Preterm"

Babies born between 34–36 weeks are considered late preterm and, while they generally do well, may need some additional medical support for feeding and temperature regulation. The lungs continue maturing throughout this period.

Weeks 37–40: Full Term

Pregnancies reaching 37 weeks are considered "full term." From week 37 onwards, the baby is ready to be born at any time. The average UK pregnancy lasts 40 weeks and 3 days from the first day of the last period.

4. Common Symptoms by Trimester

5. Key Fetal Development Milestones

6. Key Antenatal Appointments (UK)

In the UK, the NHS recommends a series of appointments throughout pregnancy. First-time mothers are typically offered 10 appointments; those who have given birth before are offered 7. Key appointments include the booking appointment (8–10 weeks), the dating scan (11–14 weeks), the anomaly scan (18–21 weeks), and a 36-week appointment to check the baby's position.

7. When to Seek Medical Advice Immediately

Contact your midwife or go to your nearest maternity unit immediately if you experience heavy bleeding at any stage, severe abdominal pain, your baby's movements significantly slow or stop after 24 weeks, signs of pre-eclampsia (severe headache, visual disturbances, sudden swelling), or signs of labour before 37 weeks (regular contractions, waters breaking).