First trimester · Week 4 of 40
4 weeks pregnant: the size of a poppy seed
The embryo implants in the uterine lining. The placenta and amniotic sac begin to form.
Reviewed against ACOG, NHS and WHO guidance · Updated November 2026
Typical size
About 1 mm
10% of the way to your due date
Your baby this week
The embryo implants in the uterine lining. The placenta and amniotic sac begin to form.
Your body this week
A missed period. A home pregnancy test can now usually detect hCG.
Tip for week 4
Book your first prenatal appointment with a doctor or midwife.
Carry this guide in your pocket
Mamivaa is a free pregnancy app with a week-by-week tracker, vitamin and water reminders, a food scanner and a baby checklist.
Start freeThis guide offers general information and does not replace advice from your doctor or midwife. If something feels wrong, contact your provider or emergency services.