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Baby Development Week by Week

Here’s a quick and easy guide to the remarkable development that takes place during a typical pregnancy. You can also read information on the stages of pregnancy on the Ministry of Health's website

First trimester – to week 13
Second trimester – weeks 14 to 27
Third trimester – weeks 28 to 42

First trimester

Your first trimester begins with your last menstrual period. It includes conception, which usually happens during week two or three.

Week 5

  • Baby is the size of an apple seed
  • Heart is forming
  • Neural tube is developing

Week 6

  • Baby is the size of a pea
  • Arm and leg buds are developing
  • The vital organs have formed

Week 7

  • Baby is the size of a bean
  • Muscles, bones and brain are growing
  • First movements occur, but you won’t feel them

Week 8

  • Baby is 2cm
  • Fingers and toes have formed
  • Baby has a paper-thin skin

Week 9

  • Baby is 2.5cm
  • Ears are developing and eyelids have formed
  • Organs are starting to work

Week 10

  • Baby is 3.5cm
  • Weight is approximately 9 grams
  • Genitals are developing

Weeks 11 to 12

  • Baby is as long as a thumb
  • Can kick and move
  • Has fully formed organs, fingers and toes

Week 13

  • Baby is 7.5cm

    Can swallow

    Fingerprints are developing

Second trimester

This is a time of faster growth. You’ll start to feel tiny flutters as baby moves. Some of your favourite clothes might be getting a little tight as baby shifts up out of your pelvis.

Week 14

  • Baby is 7-10cm
  • That’s about the size of half a banana
  • Fingerprints have formed

Week 15

  • Baby is 10cm
  • Sucking, swallowing and gripping are taking place
  • Baby is covered in soft, downy hair

Week 16

  • Baby is 11cm
  • Fingernails have formed
  • Joints and limbs working

Week 17

  • Baby is 12cm
  • Breathing practice begins
  • Baby is putting on weight

Week 18

  • Baby is 14cm
  • Thumb sucking may start
  • Baby moves arms and legs, and kicks

Week 19

  • Baby is 15cm
  • Has a heartbeat that’s much faster than yours
  • Genitals are fully formed

Week 20

  • Baby is 16cm
  • Has hair on head
  • Brain nerves are developing

Week 21

  • Baby is developing eyebrows and eyelashes
  • Digestive system is working
  • Vernix, a white creamy wax-like protective layer, is coating baby’s skin

Week 22

  • Baby is 19cm
  • Can hear voices

Week 23

  • Baby is 20cm
  • Eyes are fully formed but have no colour yet
  • Baby has wrinkled skin

Week 24

  • Baby is 21cm
  • Lungs are getting ready to breathe

Week 25

  • Baby has a pattern of wake and sleep times

    Recognises mother’s voice

    Continues to move and somersaults when mother is still

Week 26

  • Baby is 23cm
  • Responds to touch
  • Practices breathing

Week 27

  • Baby is now 25cm when tucked up and 38cm with legs extended

    Eyes begin to open

    Finger and thumb sucks are taking place

Third trimester

During the final trimester baby starts to look more rounded as fat builds up ready for the birth. The lungs are also preparing for the big day. Movement during baby’s wake times will continue to tell you that your baby is energetic and therefore feeding well.

Week 28

  • Baby weighs around 1.1kg or 2.5 pounds
  • Weight gain is filling out baby’s shape
  • Still busy moving

Week 29

  • Baby is 26cm from head to bottom
  • Weighs around 1.3kg or 3 pounds
  • Eyes open

Week 30

  • Baby is 43cm
  • Reacts to loud sounds
  • Brain is forming the grooves seen in an adult brain

Week 31

  • Baby doesn’t grow much more but does gain weight
  • Is active on some days and quiet on others
  • Can tell whether it is dark or light

Week 32

  • Baby can see and hear
  • There’s less room to move now
  • You may feel baby squirming and wriggling

Week 33

  • Baby is 45cm
  • Weighs about 1.6kg (3.5 to 4 pounds)
  • Could have full head of hair

Week 34

  • Baby is about 46cm and might weigh 2kg (4.5 pounds)
  • Big moves happen less often, mainly lots of little ones
  • May settle in a head down position

Week 35

  • Baby might be 48cm and approximately 2.3kg
  • Lungs are almost fully developed

Week 36

  • Baby might be 50cm and about 2.7kg or 6 pounds
  • Baby’s head will descend into your pelvis

Week 37

  • Baby might be 51cm and about 2.9kg or 6.5 pounds
  • Gaining weight steadily
  • There’s no longer enough room for baby to somersault

Week 38

  • Baby might appear less active
  • Has lost the downy hair and vernix coating
  • Baby might sit low in your pelvis

Week 39

  • Baby is fully developed
  • Continues to gain weight

Week 40

  • Baby is ready to be born
  • Average weight is 3.4kg or 7.5 pounds
  • Average length is 51cm
  • Babies can be bigger or smaller, longer or shorter

Week 41

  • Waiting for baby to arrive
  • Baby feels safe and secure inside


National Women's Health
Phone: 09 307 4949
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