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Fetal Development: At a Glance

What's happening when inside that growing bump!

Posted: 15 June 2009
by ThinkBaby

Pregnancy is an experience like no other, but it can be hard to imagine just what’s going on in there! Our guide explains how your baby develops week by week, so you can savour every moment. And don't forget you can sign up for FREE fetal development newsletters by email for every week of you pregnancy…

Weeks 0-4: Approx 0.2mm long
It’s very early days, so you’ve probably got no idea that you’re pregnant, but there’s already plenty going on in there. Fertilisation occurs within a few days of ovulation, and at this very moment the embryo is a bundle of around 200 cells. Over the next 96 hours it floats into your womb, and in another two days it implants itself there. It starts to grow inside a fluid bag (called a gestation sac), which by week four is around 3mm in length. The eyes and brain start to develop, and the heart begins to beat after just 22 days.

Weeks 5-6: Approx 4-6mm long
The embryo’s heart is so small that it’s still hard to hear it beating. If you had an ultrasound scan now (which is unlikely unless there are problems or expected problems), you would be able to see the blood starting to circulate. The umbilical cord has formed, ready to supply all necessary nutrients from you to your baby, and the neural tube that will later evolve into the spine and brain is developing.

Week 7-8: 9-18mm
It’s likely that you’ve noticed you’re pregnant by now, and while you’re getting to grips with the exciting news, the embryo is taking shape. Little arms and legs are forming, with fingers and toes becoming noticeable (albeit with webbing!). External genitals are also appearing, and eyelids, the lower jaw, teeth pods and the nose are continuing to develop.
At only eight weeks, the heart and nervous system is fully formed. The kidneys and bladder have developed, while the lungs are just starting to form. The embryo can now be seen to have a definite head and tail – yes, a tail! – and starts to make tiny movements. If you’re at this stage and can’t feel them, don’t panic: the movements are so tiny that you won’t be able to feel them. There’s plenty of time for that later.

Week 9-10: 2-4cm, 5-8g
That little tail has gone now, and your nine-week-old embryo’s body has lengthened. The eyelids and little nostrils form, and the internal sex organs have developed – but you won’t quite be able to tell the sex yet; you’ll have to wait another few weeks.
Fanfare, please – by week 10 you can start officially calling your little one a foetus! Most of the internal organs (liver, kidneys, intestines, heart and lungs) are rapidly developing, while the skeleton has also begun to form. The ankles and wrists are in place, the webbing on the fingers and toes is disappearing, muscles are developing and the little hands can now move towards each other – bless. You still won’t be able to feel these small movements; the foetus may be swimming away happily in the amniotic fluid, but it’s so small that it won’t bump into the sides.

Weeks 11-13: 4.5-6cm, 10-18g
Now’s the time when your foetus starts to look more like a proper baby. By 11 weeks the ears and sense of hearing are developing, as are the tooth buds and finger- and toenails. The heart is fully developed, and by week 12 the head has grown so much that it already has a circumference of more than 6cm.
If you have an early dating ultrasound scan you’ll be able to see the lungs move, as the amniotic fluid flows in and out. By the 13th week, the foetus is floating in 100ml of amniotic fluid and breathing, swallowing and sucking reflexes are starting to develop at a rapid pace.

Weeks 14-15: 8.5-9.5cm, 45-80g
If you have your dating scan around this time, you may be able to see the foetus yawning, rubbing her eyes, blinking or sucking her thumb. You will even be able to hear the heartbeat on your doctor’s Doppler instrument, so it’s a great time to get closer to your baby. The scan may also be able to indicate the sex of your baby, but remember this can still be inaccurate.
Boy or girl, the foetus is moving around quite a bit now. The skeleton is developing even more, and by week 15 your little one’s face is well defined and will be putting on a lot more weight. Those little fingernails are growing, and the legs and arms will soon be more coordinated when they move.

Week 16-17: 11.5-13cm, 110-150g
By this point, the foetus is swimming around in 180ml of amniotic fluid. Fine, translucent hair called lanugo has grown on her skin (don’t worry, this is normal, and is thought to help regulate her body temperature) while the eyelashes and eyebrows are becoming prominent. The eyes will stay closed for around another 12 weeks, but your baby can already respond to light and sound from within the womb, and make facial expressions. The rest of the body is catching up with the oversized head, with the sex organs having fully developed and the fingers and toes now moving freely. In fact, that goes for the foetus in general, who will only lie still for around six minutes at a time.
Oh, and there’s an awful lot of stress on your kidneys at this time – the foetus wees every 40 to 45 minutes into the amniotic fluid, which you have to process through your own excretory system. Nice, huh? Just as well it’s only for a few more months…

Week 18-19: 14-15cm, 200-260g
That lanugo hair has completely covered your foetus now. There are taste buds in there, and even blood cells have started to form within the bone marrow. To keep warm, the foetus is also developing brownish deposits of fat.
Are you being kept awake by wriggling? That’s because your baby is mostly rocked to sleep by your movements during the day, meaning she fancies coming out to play when you’re resting at night. Those movements are getting stronger, too, because the muscles are developing, so there may be bumps against the side of the amniotic sac. If she is keeping you awake, take heart from the fact that at least feeling her move is an exciting experience, and is reassuring that she’s growing as she should be! And don’t worry if you haven’t felt anything yet – every pregnancy is different, and she’ll let you know she’s in there sooner or later!

Weeks 20-21: 16-18cm, 320-390g
Your foetus’s neck is getting stronger now – and it needs to in order to handle the weight of the rapidly developing brain! Those ears can hear everything that’s going on outside, too, and there’s a protective oily or waxy layer (vernix) on the skin.
Most women will be offered a scan now, where you’ll be able to hear your baby’s heartbeat and see her moving around. You’ll also be able to confirm whether you’re having a boy or a girl, if you want to. It’s a very exciting time!

Weeks 22-23: 19-20cm, 460-540g
Patches of hair are growing – such as eyebrows – and there’s possibly hair on her head, too. The lungs are now producing surfactant, which helps them to stay expanded after a breath.

Week 24-26: 21-23cm, 630-820g
Hooray! No longer a foetus, your little one is officially a baby – and looking more like one too, as there’s fat being stored beneath the skin. All the organs are in place and working hard, especially those tiny lungs, which are guzzling amniotic fluid – you may even feel her hiccuping. All the senses are developing, including smell, and your baby is aware of what’s happening – expect a vigorous kicking upon hearing a familiar voice!

Weeks 27-29: 24-26cm, 920g-1.15kg
You’re in the home straight now – welcome to the third trimester. You’ll probably be glad to hear it, as your baby is now so much bigger and will be struggling for space in there as her body finally catches up with the size of the head. The little lungs are working away at a constant rhythm, the brain is developing, and the eyes are starting to open. If born now, she has a very good chance of survival.

Week 30-31: 27cm, 1.3-1.5kg
Thought your baby’s growth spurt was nearing an end? Not quite. During this trimester your baby will triple in size, putting down stores of fat and losing the fine layer of lanugo in the process. Those eyes are now open and will blink if they pick up light through the womb.

Week 32-33: 28-29cm, 1.7-1.9kg
Your baby will continue to respond to familiar voices and songs, so talking or singing to your bump can be a great bonding activity. Try to get dad involved too, so she’ll recognise him! Her sleeping cycles will be extending to around half-hour rests followed by 30 minutes of activity.
By now, many babies have moved into the head-first position (or presentation) ready for labour, although many are still breech – bottom or feet first – or sideways. If your baby hasn’t turned yet, don’t worry, there’s still plenty of time.

Weeks 34-35: 30cm, 2.1-2.3kg
Your baby’s skin is now gaining a proper colour, instead of its previous translucent hue, and its wrinkly texture is becoming smoother. And if you’re at the 34-week mark, you don’t need us to tell you that she’s stretching and kicking in every direction!

Weeks 36-37: 2.5-2.7kg
Your plump little baby’s activity levels alternate between around 40 minutes of sleep and 40 minutes of playing. Those chunky hands can grip properly, the lungs are working harder and harder, and the swallow reflex is gobbling down around 750ml of amniotic fluid a day in preparation for drinking milk.

Weeks 38-39: 2.9-3.2kg
Every baby and every pregnancy is different, but at any point from now until week 42, your little one will become fully matured or ‘full term’ – ready to enter the world. She will be putting on around 14g fat a day and, as you’ve probably noticed, hasn’t got a lot of room to move about.
Her bowel is full of a hard, green-black substance called meconium, which will become softer and be pooed out once she is born. Although by this point your baby’s growth has slowed down, all the extra hormones raging around your body will affect your little one and, boy or girl, the genitals will be comparatively very big. There’s no need to worry – they’ll go back to normal after the birth!

Weeks 40+: 45-53cm long at birth, average 3.5kg (from 2.5–4kg is common)
Your baby is full-term now, which means he or she is completely developed – brain, nervous system and all. The head hair will be around 2-4cm long and the sleeping cycles have lengthened again, to 60 minutes each of activity and rest.
You’re no doubt looking forward to your due date, but unless yours is one of the rare five per cent of timely babies, you can expect to miss that. The good news, however, is that most babies arrive within 14 days of this. Be assured that your baby will decide when the time is right for her, and will send labour signals to your womb, which will start to prepare itself for the task ahead. Then it’s show time!

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