How accurate is a due date?

A due date is an estimate. Only about 1 in 20 babies arrive on it; most are born within two weeks either side.

Can my due date change?

Yes — early ultrasound measurements can refine it. Later scans usually do not shift an established date.

Are these baby-size comparisons exact?

The fruit and vegetable comparisons are friendly approximations of average length. Every pregnancy is unique — your baby may be a little larger or smaller.

Week 3 of pregnancy

3 Weeks Pregnant

First trimester

Baby size

About the size of a pinhead

0.1 cm

What's developing

A fertilized egg (zygote) is now a tiny cluster of cells dividing rapidly as it moves toward the uterus.

Common symptoms this week

Mild bloating
Breast tenderness
Fatigue
Nothing noticeable yet

This is an estimate for information only — only about 1 in 20 babies arrive on their due date. Always confirm dates with your midwife or doctor.

How big is a baby at 3 weeks?

Baby growth chart

Typical fetal length and weight from week 8 to week 40.

Weeks 8–40

Length

Weight

grams

Typical averages based on Hadlock / ACOG references. Weeks 8–20 use crown–rump length; weeks 21–40 switch to crown–heel — that's the small jump you'll see mid-chart. Real babies vary — your ultrasound is the reliable number for you.55

Frequently asked questions

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