See how your baby grows week by week. This free baby development calculator shows a size comparison and the key things developing at each week of pregnancy, from week 4 to 40.
A general, week-by-week look at how a baby typically develops. These are typical milestones, not predictions — every baby grows at its own pace. For anything about your pregnancy, your midwife or doctor is the best guide.
Baby’s week
weeks
Enter or slide to a week from 4 to 40 to see roughly how big the baby is (with a familiar size comparison) and what’s typically developing that week.
About the size of
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Choose a week
Typically developing now
Trimester
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Weeks to 40
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General educational information about typical development, not medical advice. Speak with your midwife or doctor about your baby and pregnancy.
Use this free baby development week calculator to explore how a baby grows week by week. Pick a week to see roughly how big the baby is and what’s developing at that stage.
What this baby development calculator does
This baby development week calculator is a baby-focused, week-by-week guide. For any week, it shows a familiar size comparison and the key things developing then — from the first heartbeat to the senses and final growth before birth.
Development by trimester
Trimester
What’s happening
First (weeks 1–13)
Heart begins to beat; organs, limbs and facial features form.
Second (weeks 14–27)
Movements felt; senses develop; the baby can hear sounds.
Third (weeks 28–40)
Rapid growth; lungs and brain mature; baby settles for birth.
Size comparisons week by week
A fun way to picture growth: around week 8 the baby is roughly the size of a kidney bean, by week 20 about a banana, and by week 40 around a small pumpkin. The calculator shows a comparison for every week.
Want where you are in pregnancy instead?
This page focuses on the baby’s development. To see your trimester, month and progress, use the Pregnancy Week Calculator, or work out your current week from a date with the Pregnancy Calculator.
How to use the baby development calculator
Enter a week (4–40) or use the slider.
See the size comparison for that week.
Read what’s typically developing then.
Baby Development FAQ
How big is my baby each week?
The calculator gives a familiar size comparison for every week — for example a kidney bean around week 8, a banana around week 20, and a small pumpkin by week 40.
When does the heartbeat start?
The heart begins to beat around week 5–6, and it can often be seen on an ultrasound from about week 6.
When can the baby hear?
Hearing develops through the second trimester, and from around week 18–24 the baby can respond to sounds, including your voice.
When will I feel the baby move?
Many people first feel movements between weeks 16 and 22, often a little later in a first pregnancy.
How is this different from a pregnancy week calculator?
This tool focuses on the baby's development and size; the Pregnancy Week Calculator focuses on your stage — trimester, month and progress.
Is the baby development calculator free?
Yes, this baby development week calculator is completely free, needs no sign-up, and works instantly in your browser.
