The third trimester (weeks 28 to 40) is a time of rapid growth and remarkable development for your baby. What started as a flutter in your belly has become a stretching, kicking, hiccupping little person who is almost ready to meet the world.
So what exactly is happening during these final weeks? From size milestones to sensory development, here’s a week-by-week breakdown of your baby’s progress in the third trimester, with expert-backed insight to help you feel informed and connected.
Baby Growth Timeline: Weeks 28 to 40
Week 28
- Baby is about 37.5cm long and weighs around 1kg
- Eyes can now open and close
- Sleep cycles begin, including REM (dream) sleep
- The brain is rapidly developing
Week 30
- Baby is around 39.9cm and 1.3kg
- Bone marrow takes over producing red blood cells
- Lungs continue to develop but aren’t fully mature yet
Week 32
- Baby measures about 42cm and weighs around 1.7kg
- Toenails are visible, and skin is less wrinkled
- Baby may start to position head-down
Week 34
- Weight increases to 2.3kg, and length is around 45cm
- The nervous system and immune system continue maturing
- Baby is practising breathing, sucking, and swallowing
Week 36
- Baby is about 47.5cm and weighs 2.7kg
- Fat stores increase for temperature regulation after birth
- Most babies are head-down by now
Week 38 to 40 (Full Term)
- Baby weighs around 3 to 4kg and measures 48 to 52cm
- Lungs are mature and ready for life outside the womb
- Vernix (the white protective coating) and lanugo (fine hair) start to shed
- Movements may feel different but should remain regular
How Does Baby Prepare for Birth?
- Settles lower into your pelvis (called engagement)
- Builds up fat and muscle for life outside the womb
- Strengthens sucking and breathing reflexes
- Practices opening and closing hands
- Responds to light, sound, and even your voice
They’re busy getting into the best possible position for birth.
Baby Movements in the Third Trimester
You should feel regular movements right through to birth. This includes kicks, rolls, jabs, and stretches.
If movements slow down or change noticeably, contact your midwife immediately. Never wait.
How Is Baby Monitored in the Third Trimester?
Your midwife will:
- Measure your bump (fundal height)
- Check baby’s position (head-down, breech, transverse)
- Listen to baby’s heartbeat
- Offer extra scans if there are concerns about growth or movement
What If Baby Is Measuring Big or Small?
If your bump measures significantly above or below the expected size for your stage, your midwife may refer you for a growth scan. This checks:
- Baby’s estimated weight and size
- Placenta function
- Amniotic fluid levels
It’s not unusual for babies to be slightly above or below average, but extra checks help ensure everything is progressing well.
Midwife’s Top Tips
- Start tracking movements daily from 28 weeks
- Talk to your bump, your baby can hear and respond to your voice
- Eat well and rest when you can to support your baby’s development
- Attend all antenatal appointments to keep an eye on baby’s progress
- Don’t compare bump sizes - every pregnancy is different
The third trimester is when your baby goes from a tiny newborn-in-the-making to a fully grown little human ready for life outside the womb. Each week brings new developments and increased connection as you feel their movements and see their growth reflected in your changing body.
Trust your instincts, stay in tune with your baby’s patterns, and reach out to your midwife if you ever need reassurance. You’re doing an amazing job.
