Foetal development timeline

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The transformation from a tiny fertilized egg to a fully formed baby is awe-inspiring. Here’s a look at the major milestones babies in utero typically reach, from conception to birth, and approximately when they happen.

First trimester

3 weeks

Your baby-in-the-making is a ball of cells called a blastocyst. The blastocyst already contains a full set of DNA from you and your partner, which determines sex, eye colour, and other traits.

4 weeks

The ball of cells has officially become an embryo and is about the size of a poppy seed. Over the next six weeks, all of your baby’s organs will begin to develop, and some will start to function.

5 weeks

Your baby’s tiny heart begins to beat – at twice the rate of yours. His entire “body” is only about the size of a sesame seed.

6 weeks

Facial features (like eyes and nostrils) are beginning to form, and little buds appear where arms and legs will develop.

8 weeks

Arms and legs are growing, and your baby now has little fingers, as well as a nose and upper lip. He’s moving quite a bit now, but you won’t feel it. He’s about 16mm long and weighs hardly anything – 11 grams.

9 weeks

Eyes have developed, though your baby’s eyelids are fused shut for now. He’s lost her “tail” and is starting to look more human.

10 weeks

The embryo has become a foetus. His vital organs – such as kidneys, intestines, brain, and liver – are starting to function. Tiny fingernails and toenails are starting to form.

11 weeks

Your baby is almost fully formed. Her bones are beginning to harden, and her genitalia are developing externally.He can hiccup, though it’s too soon for you to feel it.

12 weeks

You can hear your baby’s heartbeat at a prenatal check-up. (You may already have heard it at an early ultrasound.) Your baby’s just over 5cm long and weighs about 14grams.

Second trimester

14 weeks

Your baby’s kidneys are producing urine, and he releases it into the amniotic fluid. He can make facial expressions and may have discovered thumb-sucking.

15 weeks

Your baby can see light that filters in from outside your womb, even though his eyelids are still shut.

16 weeks

Your baby’s sex may be detectable at your mid-pregnancy ultrasound, which typically happens between 16 and 20 weeks.

18 weeks

If you haven’t felt your baby move yet, you probably will in the next few weeks. It’ll take a couple of weeks longer for other people to feel your baby’s movements from the outside.

19 weeks

Your baby can hear your heartbeat and sounds that come from outside your body, such as your partner’s voice. His skin is wrinkly and is covered by a protective, waxy coating. He measures about 15cm from head to bottom and weighs about 250g.

23 weeks

Your baby’s sense of movement has developed, so he can feel the motion if you dance. His sense of hearing continues to improve. You may sometimes be able to see him squirming under the surface of your belly.

24 weeks

Your baby’s taste buds are developing. His brain is growing very quickly, and his hair may be growing, too. He’s almost a foot long and weighs around 500g.

27 weeks

Your baby’s lungs are developing but won’t be fully functional for several more weeks. He’s “practicing” for life on the outside by inhaling and exhaling amniotic fluid, sleeping and waking up at regular intervals, opening and closing his eyes, and sucking his fingers.

Third trimester

28 weeks

Your baby may be dreaming. He has eyelashes, and his eyesight is improving. Billions of neurons are developing in his brain. He weighs about 1kg and is about 38cm long, head to heel.

32 weeks

He’s grown cute little fingernails and toenails. Your baby is also starting to plump up in preparation for birth. He’s almost 43cm long (head to heel) and weighs about 1.7kgs.

34 weeks

Your baby’s lungs and central nervous system are continuing to develop. His skin has become soft and smooth, and he’s filling out and getting even rounder. He’s almost 45cm long and weighs about 2kgs.

37 weeks

Your baby is now considered “early term.” Babies born now usually do well, but ideally he’ll stay in your womb for a couple more weeks to give his brain and lungs time to fully mature.

39 weeks

Your baby is now considered full-term and is ready for life outside the womb. The average weight of a new-born is about 3.4kgs, and the average length is about 50cm.

41 weeks

You’ve passed your due date and your baby is now considered “late term.” (If you’re still pregnant at 42 weeks, he’s “post term.”) Your baby’s health may be monitored with tests such as a nonstress test or biophysical profile. To avoid complications, your doctor will probably talk to you about inducing labour in the next week or two.

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