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.
Two 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.
Three 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.
Four weeks – The tiny embryo is now about the size of a poppy seed.
Five 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.
Six 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.
Seven 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.
Nine weeks – The embryo is about the size of an olive and starting to look human. The ‘tail’ has been lost and there is a more obvious head. 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.
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.
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.
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.
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.
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 and her fingerprints are forming. If she was born now she could survive with proper care.
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 powerful kicks under her rib cage. 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).
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 nearly 3kg (6 lb 6 oz) and will be nearly 50 cm (19 in) from head to toe.
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.