3 weeks pregnant: the size of a pinhead
The fertilised egg (zygote) divides rapidly into a ball of cells called a blastocyst and travels to the uterus.
Reviewed against ACOG, NHS and WHO guidance · Updated November 2026
Typical size
About 0.1 mm
8% of the way to your due date
Your baby this week
The fertilised egg (zygote) divides rapidly into a ball of cells called a blastocyst and travels to the uterus.
Your body this week
Most women feel nothing yet. Some notice light spotting when the embryo implants.
Tip for week 3
Keep taking folic acid — it lowers the risk of neural tube defects like spina bifida.
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.