If you want to know what’s happening to your developing baby this section will tell you what you need to know. How long, how heavy, which bits develop when and what your baby can do.

This guide to the development of your baby is a week by week guide over a 9 month period divided into 3 stages or trimesters. It gives the developments, movements and other events with a guide to the average size of the baby at each stage (length and weight).

first trimester

2-4 weeks (starting from conception)

After floating in your uterus for a few days, the fertilised egg embeds itself in the lining of the womb. The amniotic sac and placenta are just beginning to grow around it, and already specialised layers of cells have developed which will form your baby’s vital organs, nervous system, bones, muscles and blood. By the end of this period the embryo is no longer a cluster of cells, it is a prawn-shaped and areas of tissue can be seen which will go on to form the spine.

Length: 4mm
Weight: Less than 1g

5 weeks (3 weeks after conception)

The developing embryo has 3 layers: In the top layer (the ectoderm) the neural tube will form and develop into the nervous system (brain and spinal cord). The spine, brain, muscles and bones are starting to form, and already a heart is developing.

6 weeks

Your baby now has a defined head and body, with limb buds appearing where the arms and legs will develop. The first heart beats begin. The baby is now an embryo. This is a most critical time in the baby’s development. During the next 2 weeks the brain, spinal cord, heart and most internal organs will develop. Tiny buds for milk teeth are forming in the jaw. The first signs of the kidneys are apparent and the umbilical cord starts to develop.

Length: 6mm

7 weeks

Your baby is growing fast. The brain, nostrils, lips and lenses of the eye are visible. The baby’s head is bent forward over its chest, the main airways to the lungs (bronchi) have developed.

8 weeks

Your baby now has all its internal organs – lungs, intestines, kidneys, liver and sex organs – though they’re not yet fully matured. Its face starts to look more recognisably human, with a jaw, mouth and nose, and eyelids starting to form. Your baby is already able to more around, though it’s too soon for you to feel anything.

Length: 2.2cm

9 weeks

Your baby is starting to look like a tiny human being. A neck is forming, and the arms, legs, fingers and toes are all longer. Tiny ears are visible and your baby can move individual limbs. Cartilage and bones are beginning to form.

Length: 2.5cm
Weight: 3g

10 weeks

Your baby is the about the size of a radish. The placenta is maturing and begins to produce the pregnancy hormone progesterone. Your baby is attached to the placenta by the umbilical cord, through which blood vessels pass nutrients and oxygen to the baby and remove waste products. The eyelids have now fused over you baby’s developing eyes, and the ankles and wrists are clearly defined, with tiny fingers and toes.

Length: 4.5cm
Weight: 5g

11 weeks

Your baby moves quite a lot (even though you can’t feel it yet). This is a period of rapid growth, and your baby starts to hold its head up with the chin away from the chest. It’s now possible to tell whether it’s a boy or girl from the external genital organs.

Length: 5.5cm
Weight: 10g

12 weeks

By the end of this week, your baby will be fully formed. Your baby’s internal organs are now fully developed, though they still need time to grow and mature, so there’s far less risk of abnormalities caused by infections like rubella. Your baby can stretch, roll, yawn and wiggle its fingers.

Length: 6.5cm (2.5 in)
Weight: 18g

second trimester

13 weeks

Your baby is now fully formed and looks like a tiny human being. He/she now needs to do a lot of growing and maturing. Up until now his head has grown more quickly than his body, but now the body starts to catch up.

Length: 7.5cm (3.5 in)
Weight: 30g

14 weeks

Eyebrows are growing, and swallowing and sucking movements begin as your baby practises these essential survival skills. The placenta is now fully responsible for nourishing your baby. Your baby can now hiccup and flex his/her legs.

15 weeks

Your baby’s eyes are tightly closed beneath the sealed eyelids. As well as swallowing he makes the ‘rooting’ movements he’ll use to latch on to your breast for feeding after birth. He can grasp the umbilical cord. Blood vessels are visible under the skin.

16 weeks

Your baby’s bones have already developed and they are now starting to harden. A fine downy hair called lanugo starts to appear over your baby’s skin. Vigorous movements continue – he/she may even find and suck their thumb.

Length: 16cms (6in)
Weight: 135g

17 weeks

Your baby is about the size of a large banana. It is floating in warm amniotic fluid, and there is still plenty of room for vigorous movement, somersaults and rolls.

18 weeks

Your baby makes breathing movements and takes amniotic fluid in and out of the lungs. Your baby starts to accumulate fat, though still looks very thin. The skin is so translucent that the blood vessels can clearly be seen through it.

Length: 20cm (8 in)

19 weeks

Buds for your baby’s permanent teeth are developing behind the buds already formed for milk teeth.

20 weeks

The eyebrows and eyelashes are starting to grow. Special glands in your baby’s skin start to produce a thick white substance called vernix, which helps to protect and moisturise the skin.

Length: 25.5cm (10 in)
Weight: 340g

21 weeks

Some hair is growing on your baby’s head, and you’re transferring antibodies from your blood to his through the placenta to help protect him/her from infection in the early months of life.

22 weeks

Your baby’s eyelids and ears are well formed. Your baby starts to swallow amniotic fluid, partly to practise swallowing and help the development of the digestive system, and perhaps to gain essential nutrients too.

23 weeks

Your baby will probably have a sleeping pattern. Growth is slowing down now as he/she starts to mature. The skin is still wrinkled needs to deposit fat. Your baby looks like he/she will at birth.

24 weeks

Your baby has a strong sense of touch. The hearing is developed and will be able to hear and may even respond to music and voices. Your baby’s vital organs have matured. Although the lungs are not yet ready to cope with independent breathing, some babies born prematurely at this stage have been known to survive, with the best neonatal care. Any baby born at 24 weeks or beyond is ‘viable’, which means the birth has to be registered.

Length: 33 cm (13 in)
Weight: 570g

25 weeks

Your will probably now know when your baby sleeps and wakes. Your baby has less room for movement now. He/she continues to swallow amniotic fluid – and to urinate into it.

26 weeks

The skin is gradually thickening and appears less translucent, and your baby continues to lay down fat stores that will help to regulate its body temperature after birth.

27 weeks

Around now your baby’s eyes open, and he can blink. The retina at the back of the eye is developed and it’s likely that your baby is now sensitive to light.

28 weeks

He continues to gain weight, and space is restricted.

Length: 37cm (14.5 in)
Weight: 900g

third trimester

29 weeks

Your baby’s brain increases in size and complexity. They can distinguish between sweet and sour tastes, and his/her hearing is now acute. You’ll notice him/her jumping or startling to loud noises.

Weight: almost 1lb

30 weeks

He/she feels the rhythmic squeezes of ‘Braxton Hicks’ contractions, and his/her heart is pumping at a rate of 120 and 160 beats each minute.

31 weeks

The lungs still need to develop surfactant, a substance that allows your baby to breathe unaided.

32 weeks

At this stage your baby’s swallowing reflex matures, but if born at this time he would need special care in an incubator. His growth and heartbeat are checked at each antenatal visit.

Length: 40.5cm
Weight: 1.6Kg

33 weeks

Your baby is fully formed and now only needs time to gain weight and plump up ready for birth. Your should feel prods and kicks at least ten times a day. Your baby’s lungs secrete surfactant (a soapy fluid). This lubricates and keeps them open, ready for breathing.

34 weeks

Around now your baby will probably turn to the head down position. He will stay there until the birth. A breech (feet first) baby may turn later.

35 weeks

Your baby’s going through a rapid growth phase, and may gain up to half a pound each week. Through the wall of your tummy, your baby’s open eyes can differentiate between darkness and light.

36 weeks

Around now your baby may ‘engage’, or drop down deeper into your pelvis. Your midwife can feel when this has happened.

Length: 46cm (18 in)
Weight: 2.5kg

37 weeks

If you’re expecting a boy, his testicles will have descended from the abdomen in to place in his scrotum. Fingernails and toenails are complete, and your baby gains an ounce a day from now on.

38 weeks

Movement is restricted to prods and knocks, and your baby may get hiccups as a result of gulping amniotic fluid. His lungs are now ready to cope with real air.

Length: 35-37cm
Weight 3-4Kg

39 weeks

His bowel contains a thick, dark sticky substance called meconium that he’ll pass as a first stool soon after birth. The lanugo (hair) has rubbed away by now, though traces may remain.

40 weeks

Fingernails are growing, perhaps long enough for your baby to be born with scratches. Plump and mature, your baby is ready for birth!

Length: 55cm (20 in)
Weight: 3-4 Kg