Complete Week-by-Week Pregnancy Guide: What to Expect Every Week

How is pregnancy dated?

Pregnancy is dated from the first day of your last menstrual period (LMP), not from conception. Your due date is calculated as 40 weeks (280 days) from your LMP. According to the NHS, only about 4% of babies arrive on their due date. You can check or confirm your dates with our due date calculator, and jump to any week of your pregnancy using the week grid below.

First trimester: weeks 1 to 12

By the end of week 12, every major organ system has begun to form. Your baby has a beating heart, and tiny fingers and toes are visible on ultrasound.

Weeks 1 to 4: Conception and implantation

The fertilised egg implants in the uterine lining around day 6 to 12. By week 4, the embryo is about the size of a poppy seed.

Weeks 5 to 8: Heart begins beating

The neural tube is forming, which is why the NHS recommends 400 micrograms of folic acid daily. By week 6, the heart beats at around 110 bpm. By week 8, the embryo is about 1.6 cm long.

Weeks 9 to 12: End of embryonic stage

From week 9, your baby is officially called a foetus. By week 12, your baby is about 6 cm long and weighs around 18 grams. Your NHS booking appointment typically happens between weeks 8 and 12. If you are unsure whether what you are feeling is typical, our pregnancy symptom checker covers the common first-trimester symptoms.

Second trimester: weeks 13 to 27

Weeks 13 to 16: Energy returns

Nausea typically eases as the placenta takes over hormone production. By week 16, your baby is about 12 cm long.

Weeks 17 to 20: First movements and anomaly scan

Most women feel their baby move between weeks 18 and 20. The 20-week anomaly scan checks your baby's organs and structures.

Weeks 21 to 27: Rapid growth

By week 24, your baby reaches viability. By week 27, your baby weighs approximately 900 grams and responds to sounds.

Third trimester: weeks 28 to 40+

Weeks 28 to 32: Brain development accelerates

At your 28-week appointment, your midwife checks blood pressure and offers blood tests. If you are Rhesus negative, you will be offered an anti-D injection.

Weeks 33 to 36: Preparing for birth

Lungs are maturing and producing surfactant. By week 36, most babies are head-down.

Weeks 37 to 40+: Full term and beyond

Per ACOG, full term is 39 to 40 weeks. NICE guideline NG207 recommends offering induction at 41 weeks. When things start, our contraction timer tracks how long contractions last and how far apart they are, and our guide to Braxton Hicks vs real contractions explains how to tell practice tightenings from the real thing.

Key NHS appointments