Pregnancy week by week: weeks 4 to 40 with sources

How this guide works

Each page in this guide covers one week from 4 to 40. It sets out what named health bodies say about how the baby develops and about the checks, tests and appointments they describe for that week. Every statement is attributed in the text to the body and year, and the full list of sources is at the end of each page. This is information, not advice: nothing here diagnoses a symptom, and the box on each page lists the symptoms that need urgent help.

Where the NHS in the UK and ACOG or the Office on Women’s Health in the US give different figures or different timings, the page shows both and says which is which. Where the sources disagree on how a week is counted, the page says so and does not pick one.

How the weeks are counted

ACOG says a normal pregnancy lasts about 40 weeks from the first day of the last menstrual period, and that pregnancy is assumed to start 2 weeks after that day, so an extra 2 weeks is counted at the beginning before the pregnancy has begun (ACOG, How Your Fetus Grows During Pregnancy, 2024). The NHS also says pregnancy is dated from the first day of your last period (NHS, "Week-by-week guide to pregnancy", 2026). On this site a page for week 12 covers 12 weeks 0 days to 12 weeks 6 days, the way the pregnancy week calculator counts.

Where the trimester lines fall

• The NHS week-by-week guide groups weeks 4 to 12, 13 to 27 and 28 to 41 (NHS, Week-by-week guide to pregnancy, 2026).
• The Office on Women’s Health defines the first trimester as week 1 to week 12, the second as week 13 to week 28 and the third as week 29 to week 40 (Office on Women’s Health, Stages of pregnancy, 2025).
• ACOG defines the first trimester as the first day of the last menstrual period to 13 weeks and 6 days, the second as 14 weeks and 0 days to 27 weeks and 6 days, and the third as 28 weeks and 0 days to 40 weeks and 6 days (ACOG, 2024).

The lines are conventions, so a person in week 13 is in the second trimester by the NHS’s and the Office on Women’s Health’s definitions and in the first by ACOG’s. The hub for pregnancy trimesters sets the three side by side, with the guides to the first trimester, the second trimester and the third trimester.

Why baby sizes differ between sources

The NHS gives a length in millimetres or centimetres for each week and compares it to a fruit, vegetable or other object. It says that up to about week 19 the baby is measured from head to bottom and from week 20 from head to heel, because the legs are curled up and hard to see in the earlier weeks (NHS, "Week-by-week guide to pregnancy", 2026). The Office on Women’s Health gives sizes in inches and ounces at selected weeks, from four to five weeks to birth, and does not say which measurement it uses (Office on Women’s Health, 2025). ACOG lists development in bands of four weeks and gives no sizes (ACOG, 2024). When the figures for a week differ, the page gives each as its source states it and does not convert one into the other.

Scans and appointments named across the weeks

NICE guideline NG201 says the first antenatal appointment with a midwife should take place by 10 weeks 0 days, a scan should be offered between 11 weeks 2 days and 14 weeks 1 day, and another between 18 weeks 0 days and 20 weeks 6 days (NICE, 2021). The NHS says you will be offered at least 10 antenatal appointments and 2 scans if this is your first baby, and at least 7 appointments if you have given birth before (NHS, Antenatal (pregnancy) care and appointments, 2026). In the US, ACOG says it is best to start prenatal care in the first trimester, and that before 10 weeks after your last period is ideal (ACOG, Prenatal Care, 2025). Your own care team gives you the schedule that applies to you.