At a glance
Which trimester is week 4?
OWH counts the trimesters as weeks 1 to 12, 13 to 28 and 29 to 40.[1] The NHS guide counts them as weeks 4 to 12, 13 to 27 and 28 to 41.[2] ACOG counts the first trimester to 13 weeks 6 days, the second from 14 weeks to 27 weeks 6 days, and the third from 28 weeks to 40 weeks 6 days.[4]
All three place week 4 in the first trimester.[1][2][4]
ACOG says pregnancy is counted from the first day of the last menstrual period, so 2 weeks are counted at the beginning when you are not yet pregnant.[4]
Your body this week
The NHS says a first pregnancy may not show until at least week 12.[3]
The NHS lists these possible early symptoms:[3]
Your baby this week
The NHS says the embryo is around 2 mm long.[3]
The NHS says the embryo is protected by the amniotic sac and fed by a yolk sac, and its outer layer will become the placenta.[3]
Visits and tests around week 4
For a healthy pregnancy, NICHD describes visits once a month until 28 weeks.[5]
Things to do this week
- The NHS says to contact a doctor or midwife to start your pregnancy care.[3]
- The NHS advises 400 micrograms of folic acid a day until at least week 12.[3]
- The NHS advises stopping smoking and alcohol, and limiting caffeine.[3]
- The NHS says not to stop medicine for a long-term condition without advice.[3]
- The NHS suggests getting checked for sexually transmitted infections.[3]
Feelings check-in
The National Institute of Mental Health (NIMH) says perinatal depression includes depression during pregnancy, and its signs last at least 2 weeks.[6]
HRSA says the National Maternal Mental Health Hotline, 1-833-852-6262, takes calls and texts 24/7 in English and Spanish.[7]
When to call your provider or get help
CDC lists urgent warning signs to watch for during pregnancy and in the year after birth, including:[8]
- a headache that will not go away or gets worse;[8]
- dizziness or fainting;[8]
- a fever of 100.4°F (38°C) or higher;[8]
- severe nausea and vomiting;[8]
- severe belly pain that does not go away;[8]
- vaginal bleeding or leaking fluid;[8]
- thoughts of harming yourself or the baby.[8]
MedlinePlus says some people spot in early pregnancy and do not miscarry, but any bleeding should prompt a call to a provider.[9]
Questions families ask
Which trimester is week 4?
OWH places week 4 in the first trimester.[1] The NHS guide places it in the first trimester.[2] ACOG places it in the first trimester.[4]
How big is the baby at 4 weeks?
The NHS says the embryo is around 2 mm long.[3]
How early can a home test show pregnancy?
The NHS says some home tests can tell after about 3 and a half weeks.[3] OWH says results are more reliable about a week after a missed period.[10]
Free from UpOnly Learning. Written for families of every faith. No outside ads and no affiliate links. Every fact on this page comes from the sources listed below. How we source and review these pages
Updated 2026-10-24: first published.
Sources
All sources were opened and checked on the date shown.
- Office on Women's Health (HHS). Stages of pregnancy. Checked October 24, 2026.
- NHS (United Kingdom). Week-by-week guide to pregnancy. Checked October 24, 2026.
- NHS (United Kingdom). 4 weeks pregnant guide. Checked October 24, 2026.
- American College of Obstetricians and Gynecologists. How Your Fetus Grows During Pregnancy. Checked October 24, 2026.
- Eunice Kennedy Shriver National Institute of Child Health and Human Development (NIH). What happens during prenatal visits?. Checked October 24, 2026.
- National Institute of Mental Health (NIH). Perinatal Depression. Checked October 24, 2026.
- Health Resources and Services Administration (HRSA). National Maternal Mental Health Hotline. Checked October 24, 2026.
- Centers for Disease Control and Prevention (HEAR HER campaign). Urgent Maternal Warning Signs and Symptoms. Checked October 24, 2026.
- MedlinePlus (National Library of Medicine). Miscarriage. Checked October 24, 2026.
- Office on Women's Health (HHS). Knowing if you are pregnant. Checked October 24, 2026.