Your pregnancy: weeks 17, 18, 19, 20

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This guide to your pregnancy tells you how your baby is developing week by week. It also tells you a bit about how your body may be changing and how you might be feeling. Remember, not all babies develop exactly like each other so this can only be a general guide: you should ask your doctor or midwife for any specific information about your pregnancy. We all know babies come in two flavours, but - as most of you won't know what flavour you're getting - for clarity we'll refer to your baby as 'she'.

How your baby is developing | Your baby's size | How your body is changing

How your baby is developing

Your baby doesn't grow as quickly during this period but builds on what she's done already. She will move more strongly, her muscles will grow and her kicking will become more forceful. Indeed, many mothers feel their baby kicking by the 20th week.

One study that looked at the relationship between when you first feel your baby move and when you give birth found that the average time between feeling the first kick and having your baby was 147 days.

Her arms and legs will reach their relative sizes in these weeks. Her neck gets stronger too, and she'll hold her head up more and more.

There are some downsides to being in an amniotic fluid bath for nine months and during these four weeks your baby develops some protection in the form of a cheesy, white cream that covers her whole body and head. Called vernix, it's made from the baby's sebaceous glands (the ones that make oily fluid to protect our skin) and acts as a waterproof layer to protect your baby's delicate skin from getting chaffed by the amniotic fluid.

It's said to make your baby pass through the birth canal more easily - it certainly can make babies feel rather slippery when they come out - and you'll see the remnants of it on your baby's skin at birth.

Your baby develops some thermal insulation as well during this period by forming a special kind of fat called brown fat. Found around the kidneys and under the skin at the base of the neck, it produces heat when needed during the newborn period.

Meanwhile, the pads on your baby's fingers and toes are beginning to develop patterns of whorls and swirls. Hopefully, she will never get finger printed in later life but this is where these unique identifiers begin. And her eyebrows and hair are now sufficiently bushy to be seen on ultrasound scan and while her eyes are still closed they look forward.

Your baby is unlikely to be born with teeth but she gets ready for them in the womb. Little buds are developed for milk teeth and behind them already are tooth buds for grown-up teeth.

If you have a girl, her womb will have grown and her vagina will start growing during this time. Her ovaries will have 6 million eggs. If you have a boy, his testes will start moving down from where they develop inside the abdomen quite high up on the back wall, to their eventual home in the scrotum.

Your baby's other organs continue to develop. Her lungs are becoming more mature with tiny air sacs called alveoli starting to develop. These sacs provide a huge area for the lungs to take in oxygen and get rid of carbon dioxide once your baby's born. Your baby will also have a fine set of vocal cords, which she may exercise, obviously silently, ready for her big moment.

Your baby won't be breathing fully because she is essentially under water in her amniotic bath, but she does have a go. In fact, she both swallows and breathes in and out the amniotic fluid that surrounds her. It is this that gives her hiccups, which early on can feel like little flutters but, later on, may be seismic movements that make your belly shake.

Your baby can swallow around a litre of fluid a day and this fluid will include her own urine that she has peed out into the amniotic fluid. Some research shows that there is a 24-hour pattern to when your baby swallows fluid and produces urine that may be related to what her mother - you - eats and drinks.

Your baby has managed to accumulate its first bowel movement. She will add to it later but meconium, as this first bowel movement is called, is made up of amniotic fluid and bits of old cells and other debris from the lining of the bowels. It is green, black and tarry. It is incredibly sticky and difficult to get off so it's worth remembering as you lovingly cuddle your newborn baby to get a nappy on her as soon as possible.

Your baby's heart is growing stronger. As time goes on it won't beat quite as fast as it did earlier on in its development.

Your baby's brain is going through a highly specialised time of development. It will be developing special senses, taste, smell, hearing and sight. The optic nerve, for example, which carries messages to the brain about what we see, is formed by over a million nerve cells that grow in to the brain. Your baby is also beginning to hear. She finds it easier to hear lower rather than higher voices.

Your baby's also developing the nerves all over her body by insulating them with a covering layer called myelin. This makes electrical impulses travel smoothly and faster down nerves.

Your baby's size

She will grow about 50mm over these four weeks to be around 15-19cm, weighing between 250g and 450g.

How your body is changing

It is difficult in discussing how you're changing not to focus on the problems, like heartburn and constipation. It is a very individual thing, pregnancy, and some women sail through it and feel fantastic. However you have felt so far, you are now half way through your pregnancy - although now you really are pregnant the weeks can seem to get longer.

• Stretching ligaments

As your womb grows and rises up and out of your pelvis, it stretches the ligaments that fix it in place. This is obviously necessary but, unfortunately, it can really hurt. You may find you get a brief stabbing pain below your bikini line; sometimes it can be a dull, achy pain, particularly if you have been on your feet all day. Called round ligament pain, it starts off feeling quite deep and can be bad enough to be worrying. If it stays, gets worse or you have any other problems like cramps, spotting or sickness then you need to see a doctor or midwife. You should always see your doctor or midwife if you have abdominal pain.

• Heartburn

Most pregnant women get heartburn, which can be anything from swallowing acidic watery stuff to burning discomfort in the middle of your chest. This is partly because pregnancy hormones relax the valve that guards the entrance to your stomach. This is where your oesophagus, a long muscular tube that runs from the back of your mouth, enters your stomach. When the valve doesn't shut properly, food and the acid that the stomach produces to digest your food sloshes back up to your oesophagus. This causes heartburn. Laying flat at night makes it worse, so raise your pillow or the foot of your bed (but obviously not both). Avoid huge meals, especially just before bed. You may want to talk to your doctor or midwife about medicines you can take.

• Bleeding gums

Many women find their gums start bleeding in pregnancy. Gums swell up in pregnancy, partly because the volume of blood you have going round your body goes up. It's important to clean you teeth well as this is a time when your gums can develop full-blown gingivitis (which is red, inflamed, angry gum disease) and flossing gently to get rid of food between teeth is essential, if fiddly. To help matters, use gentle toothbrushes and don't brush too hard; eat fruit, especially citrus food like oranges, and see your dentist more often.

You may find you also get nosebleeds occasionally. This is because the lining of your nose is also affected by the extra blood flow and gets engorged. If you blow it a bit hard or your nose gets a bit sniffy you may find you have a nosebleed.

• Constipation

Most pregnant women get constipated and it's because pregnancy hormones slow down your gut. When your gut moves more slowly you get constipated. You will find what works for you but liquorice can be good and you should be eating loads of vegetables and fruit already, which will help.

More about pregnancy health

Remember, you can chat to other mums about all the 'joys' of pregnancy on our Pregnancy Talk boards.

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