How big is the baby in week 4 of pregnancy?

In week 4, your baby is roughly 0.2 cm long and weighs about 0.1 g — about the size of poppy seed.

4 of 40 weeks

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First trimester

Week 4

The tiny ball of cells settles into your uterus.

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Baby size this week

As lovely as a poppy seed

Measurements are averages — every baby grows at their own pace.

Length 0.2 cm 0.1 in

Weight 0.1 g 0.00 oz

Your baby this week

The ball of cells is now a blastocyst, the width of a poppy seed
It implants into the uterine lining
The amniotic sac and yolk sac begin to form
The hormone hCG starts entering your bloodstream

Your body this week

Missed period

Breast tenderness

Fatigue

Light spotting

A little list for week 4

This week's to-do

Take a home pregnancy test if your period is late Keep taking folic acid Avoid alcohol and smoking Start noting your symptoms

How we measure week 4

Pregnancy weeks are counted from the first day of your last menstrual period, following standard ACOG and NHS guidance. Sizes are crown-rump length in early weeks and head-to-heel from around week 20. Your care team may adjust dates after an ultrasound.

Continue to week 5

BumpCount offers general information and does not replace advice from your doctor or midwife.

Week 4

As lovely as a poppy seed

Your baby this week

Your body this week

This week's to-do

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