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Alison Ross
Registered Midwife, DipHe, BSc (Hons) Was a midwife at Kingston Hospital and Specialist Midwife in Perinatal Mental Health.
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Antenatal care

How does my baby develop during pregnancy?

From the moment of conception, your and your partner's genes combine to create a unique human being. Over the next 40 weeks, this new baby will grow and mature ready for her arrival in the world.
In Short
Week by week during your pregnancy, your baby will be going through the most amazing journey.

For example, by week 5, your baby's heart will already be dividing into chambers. By week 24, your baby will have fingerprints!

From the minute that the sperm penetrates the egg, your and your partner’s genes combine to create a unique human being. Over the next 40 weeks, this new baby will grow and mature ready for her arrival in the world. Here is an approximate overview of this amazing process.

First-week – Sperm fertilises the egg and forms one cell smaller than a grain of sand (zygote). Over the next few days, the cell divides rapidly as it travels down your fallopian tubes and into your uterus. The fertilised egg, now called a blastocyst and consisting of 150-300 cells, implants in the womb lining.

2 weeks – The blastocyst essentially divides into two parts: cells of the outer part first form the placenta, and the inner part develops into the foetus.

3 weeks – The placenta and umbilical cord are now doing their job of delivering nutrition to the embryo, and neural tube begins to develop – this will become the brain, spinal cord and nervous system.

4 weeks – The tiny embryo is now about the size of a poppy seed.

5 weeks – The heart is already dividing into chambers, and the major organs will start to develop. Little buds that will become the limbs start to appear.

6 weeks – The brain now begins to control the movement of the muscles and organs. The embryo looks a little like a tadpole with a body and a tail.

7 weeks – The brain hemispheres are growing. There are dark spots where the nostrils and eyes are forming. The colour of the irises is now visible and the lenses are forming in the eyes. The jaw starts to show teeth buds up in the gums. Ears are starting to develop.

8 weeks – 9 weeks – The embryo is about the size of an olive and her face is forming. Your baby now has a mouth with a tongue with tiny taste buds. Hands and feet continue to form along with the fingers, toes and elbows – internal organs such as testes and ovaries start to develop. Eyelids will be completely fused over the eyes.

10 weeks – Your baby’s heart is completely developed and beats twice or three times as fast as your own heart. Your baby is able to make movements inside the amniotic fluid. You can see these on an ultrasound scan.

12 weeks – The ‘embryo’ is now officially a ‘foetus’ and is about the size of a plum. Your baby is swallowing amniotic fluid and passing urine. Wrists and ankles and tiny finger and toenails are present as well as her unique fingerprints.

13 weeks – Your baby’s genitals have begun to form but you will not be able to see whether you are having a boy or a girl yet.

14 weeks – In baby boys, the prostate gland is developing and the ovaries of baby girls are descending from the abdomen to the pelvis. Your baby’s thyroid gland starts producing important hormones for growth and metabolism and hair starts to grow. The foetus will now be about 8 cm (3 in) length.

15 weeks – Your baby ears have now developed and she can hear noises inside and outside the womb. This means when you talk to your bump your baby can hear you.

16 weeks – Now around 10 cm (4 in) long – the size of an avocado pear. The foetus can swallow, hiccup, kick and swim. Your bump will probably start to show at around this point. If you’ve had a baby before you may begin to feel movements from now. Your baby has more control over her arms and can now make a fist and reach her hands together.

20 weeks – Up until now, your baby has been measured in the scans from ‘crown to rump’ – but now she will be measured from crown to heel. At 20 weeks, she’ll be around 26 cm (10 in) long and growing fast. Your baby will be growing the waxy covering called vernix, which coats and protects her skin and makes the birth easier. You may see your baby sucking her thumb at your 20 week scan.

24 weeks – Your baby weighs around 600 g (21 oz) and is approximately 30 cm (12 in) long. All her main organs are now working, except for her lungs, which remain dormant until she takes her first breath. Your baby responds to sounds outside the uterus. You should be able to feel your baby move and make sure each day that your baby is moving.

30 weeks – At this age most babies move into a head-down position, but may not stay there before 34 weeks and are almost at their final birth length. However, their brain continues to develop at a rapid pace. You may feel generalised powerful kicks. By now your baby can open and shut her eyes. At this point, she will be around 40 cm (15 in) long and weigh around 1.3 kg (2 lb 8 oz).

34 weeks – Your baby’s toenails will have reached the tip of her toes and the umbilical cord will be about 45 cm (18 in) long. Your baby will be getting fatter now too and will weigh around 2 kg (4 lb 8oz). Her sucking reflex is developing and she will suck her fingers.

36 weeks – All the senses are now well-developed, although your baby’s eyesight will improve rapidly in the weeks after birth. She will be sensitive to hot and cold and pressure. She will be gaining a huge amount of weight – around 28 g (1 oz) a day. Your baby now weighs on average nearly 3kg (6 lb 6 oz) and will be nearly 50 cm (19 in) from head to toe. Your baby’s lungs are now developed enough to breathe alone.

40 weeks – Your baby is fully developed and ready to be born. Her hair might already be thick. Over 95 percent of babies are head-down in the uterus and will be born this way. The average weight of a newborn at term is about 3.4 kg (7 lb 5 oz) although anything between 2.5kg (5 lb 5 oz) and 4 kg (8 lb 8 oz) is considered normal.

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This article is for information only and should not be used for the diagnosis or treatment of medical conditions. Essential Parent has used all reasonable care in compiling the information from leading experts and institutions but makes no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details click here.