Your Baby's First 40 Weeks in the Womb — Week by Week
First trimester (weeks 1–12): the building phase
Everything is founded here. By week 6, a tiny heart is beating. By week 8, your baby — about the size of a raspberry — has buds for arms and legs. By week 12, all major organs exist, fingers and toes have formed, and baby is roughly the size of a plum. This is also the trimester of exhaustion and nausea for many mothers: your body is doing the heaviest construction work of the whole pregnancy.
Second trimester (weeks 13–27): the growing phase
The famous "golden trimester." Energy often returns, a bump appears, and around weeks 18–20 you'll likely feel the first fluttering movements — quickening. Baby can hear sounds by about week 18–20, practices swallowing, grows hair and nails, and by week 24 has reached an important milestone of viability. At week 27 baby is about the size of a cauliflower and can open and close their eyes.
Third trimester (weeks 28–40): the finishing phase
Now it's about growth and maturity. Baby gains roughly 200 g a week in the final stretch, the lungs produce surfactant so they can breathe air, and the brain develops rapidly. Around week 36–37 most babies settle head-down. From week 37 your baby is considered early-term; by week 40 — your due date — they're about the size of a small watermelon, and only about 1 in 20 babies actually arrives on that exact day.
How to use this guide
Find your current week in the video and watch what your baby is doing right now — then look one or two weeks ahead so the next changes don't surprise you. Mothers tell me this turns pregnancy from a waiting room into a journey you're travelling with your baby.
Common questions
Why is pregnancy 40 weeks if it's "9 months"?
Because counting starts from your last period — about two weeks before conception even happens. 40 weeks is 10 lunar months, which is why the maths never quite matches the calendar.
When will I feel my baby move?
Most first-time mothers feel movement between 18 and 22 weeks; if you've had a baby before, often earlier, around 16–18 weeks. At first it feels like bubbles or flutters, not kicks.
What if I pass 40 weeks?
Very normal — especially for first babies. Your team will monitor you and baby more closely after 41 weeks and discuss induction. Only about 5% of babies arrive on their due date.
5 Wild Things Your Baby Does in the Womb
Next, five surprising things your growing baby is already doing inside the womb.