This guide to your pregnancy tells you how your baby is developing each month. It also tells you a bit about how your body may be changing and how you might be feeling. Remember that 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 that babies come in two flavours, however - as most of you won't know what flavour you're getting - for clarity we will refer to your baby as "she".

Remember to check out our Pregnancy discussion boards here for further advice on your pregnancy - from how to combat morning sickness to where to find the best maternity jeans, other Mumsnetters have it covered. Plus, join our online antenatal club and share your experience with other Mumsnetters due in the same month as you.

Your pregnancy at 9 to 12 weeks

This is the period in which your little embryo becomes a fetus. This rise in status in the ninth week happens after your embryo loses her funny little tail and becomes altogether more baby-like. Now your baby is looking less like a jellybean... and more like a jelly baby with an oversized head. This is because your baby's head gets literally a head start in life. It takes a few weeks for the rest of the body to catch up. By the beginning of the ninth week, your baby's head will make up half of the entire crown to rump measurement (this is the measurement of your baby's sitting height). That means it is disproportionately large compared to your baby's body. However, by the end of the 12th week, the rest of the body will be catching up fast, as the length of your baby will have doubled between the 8th and 12th week. On average, your baby will be 5.4 cm long by its 12th week.

Your baby's face will be broad, with eyes widely separated and shut tight, because until 27 weeks her eyelids will be fused together. Your baby's ears are low right now but they will move up later - usually by the end of 12 weeks.

Your baby will be growing a great deal in these four weeks; by the end of the 12th week her arms will be the same size in relation to the rest of her body as they will be at birth. This means they no longer look like short flippers. Your baby's legs take longer to develop and her thighs will stay slightly stumpy for a while.

By the end of the ninth week of fetal life all babies look pretty much the same - but by the end of the 12th week some will look a lot more like little boy babies and some will look more like little girl babies. This is because testosterone, produced by the fetal testes, prompts the development of boy bits. However, in fetuses who are gong to be girls, oestrogens - which come from the fetal ovaries and also from the placenta - stop the penis from growing and turn it into a clitoris. Either way, things are still quite immature. It's unlikely that, if you had an ultrasound now, anyone would commit themselves to the sex of your baby.

Your baby is also busy developing its muscles and joints. At eight weeks she'll make some movements but by 12 weeks she will seem more purposeful and can move her arms and legs individually. By the end of the 12th week she will have developed the sorts of muscles we use to walk and run with. (These muscles look stripy under the microscope.) Once your baby has them she starts flashing them around - moving her limbs and stretching her legs, body and neck. It's thought that babies move in response to movements in the amniotic fluid that surrounds them - ripples in their own swimming pool - and also when they touch themselves, say if they brush their hand against their face or body. Ultrasound studies show that fetuses move a lot around 10 weeks but this slows down by 15 weeks, as they get bigger and your womb therefore starts being less roomy.

Your baby's internal organs, such as her liver and pancreas, are also coming along nicely. The intestines part of the gut (which, for a while, gets too big for your baby's abdomen and is pushed into the umbilical cord) comes back and starts developing specialist cells. Some of these cells will have protective, immune roles; some will produce enzymes to break down food and some will grow into the nerves that stimulate the intestine into churning up food and pushing it through the intestines. Your baby's intestines are already a hollow tube which starts moving by tightening up the muscle walls on the outside of the tube and then relaxing them. When your baby is born and starts drinking it will use this movement to push the fluid through the gut. However, it already has some work to do as babies start swallowing and take in amniotic fluid whilst in the womb. The intestines have to cope with this, which they do very well as 60 to 70 percent of the protein in amniotic fluid is digested every day.

Your baby's spinal cord - the long column of nerves that runs down the length of your back - can just about be seen now. Her brain will be forming brain cells at a rate of 250,000 a minute.

During this time your baby's diaphragm develops. This is a sheet of muscle shaped like a leaf, which separates our chest from our abdomens. For fetuses, their lungs start off right next to their stomachs, but as their lungs grow, so this leaf shaped muscle grows into the middle of the body and divides the body into a space for the lungs and a space for the stomach and bowels. Once this happens, your baby can get hiccups if her diaphragm is irritated in some way. You won't notice them now, but when your baby is bigger, her hiccups will take over your life... or at least wake you up as soon as you drop off to sleep.

How are you changing?

You will have missed your second period by now so pregnancy may be becoming even more real. Women experience pregnancy in quite individual ways but you may have some of the following:

Tiredness:

Yes, you had this during the last four weeks, but you may still have it. In fact coming up to the 12-week reprieve period it may get worse. This is tiredness like no other - not the eyelids drooping but the whole body drooping. It spreads from your feet up through your whole body and is almost pleasantly overwhelming. You can't fight it so lie down and embrace it. Of course if you have other children or a job then you probably won't be able to. Sorry.

Sickness:

See tiredness. You will either have it or you won't during these twelve weeks -and some poor women will continue to get it throughout pregnancy. The sickness is usually worse in the morning but goes on all day and, while different things work for different women, eating little and often and avoiding fatty, spicy foods is the general advice. Some women don't feel hungry - others want to eat eight meals a day.

Weight gain:

This is especially true if you have already had a baby. With indecent haste your abdomen will go "I've done this before" and spring out like an inflated balloon. Unless you have done serious abdominal work since your last pregnancy, you are doomed to look close to term by the end of 12 weeks or, at best, find you can't fit into your jeans any more. It can be depressing - illogically of course, but that's the problem with pregnancy - you know you are having a baby but you still feel fat.

Headaches:

These can last until 12 weeks but can be helped by rest. If they are bad tell your doctor.

Tender breasts:

Your breasts will still be tender and they will definitely be bigger. Don't try to cram them into your usual bra and, if you buy new ones, do bear in mind that the size your breasts are now will not be the same as their size at the end of the pregnancy. Nowhere near.

Remember, you can chat to other mums about all the "joys" of pregnancy on our Pregnancy discussion boards here.