Your baby is developing more rapidly during this time than at any other sage in your pregnancy, at week 5 the embryo is about 2mm long and could therefore be seen with the naked eye, the spinal column, the foundations of the brain and the spinal cord are beginning to develop. By week 6 the embryo is close to the size of the nail on your little finger, its immature heart has started beating, the head is beginning to form and there are indentations where the eyes and mouth will develop. There are tiny limb buds which will become arms and legs. By week 7 there are tiny depressions where the fingers and toes will grow, the intestines are formed, as are the lungs, though they are still solid. There are holes for the nostrils and the inner parts of the eyes and ears are developing. What has so far been cartilage is beginning to develop in to bone as bone cells appear.
The embryo becomes known as a fetus (Latin for 'young one') at week 8 and all the internal organs are in place, the major joints are becoming obvious and the fetus is beginning to look more human. During week 9 more human characteristics begin to develop such as the mouth and nose. Hearing has developed and by week 10 the external parts of the ears are beginning to grow. The eyes have now formed, as have the fingers and toes though they are still joined by webs of skin.
By week 11 the fetus is about 5.5cm and may weigh about 10g. The external organs are all fully formed and functioning and it is unlikely that they will be damaged by infections, chemicals or drugs after this stage. The external genital organs are formed as are the ovaries or testicles. The end of week 12 marks the end of the first trimester and although the head is large in comparison to the rest of the body the fetus is beginning to look like a baby. The face is forming and closed eyelids are distinguishable. Muscles are starting to grow and co-ordinate with the brain, joints can contract, toes can curl up and your baby can suck his thumb.

Though the first trimester is undoubtedly a crucial time for your developing baby it is also a very important time for you. You probably won't look pregnant yet, most women don't develop a bump until about 4 or 5 months but you may well feel pregnant. Common symptoms at this stage are tender breasts, nausea in the morning, or indeed at any time of the day, unpredictable emotions and of course tiredness. I was particularly lucky in my pregnancy and didn't suffer from any of these complaints; I did however find myself running to the loo about 20 times a day from very early on, in fact even before I could get a positive result from a pregnancy testing kit. This frequent urination is caused by the pressing of the uterus on the bladder as it begins to swell and usually eases off a bit after week 12 when the uterus moves out of the pelvic cavity. For a few months, that is, until the weight of your growing baby starts to press down on your bladder and you have to start running again!
Another early indication may be a change of taste in your mouth; some women experience a metallic taste while others go off certain food and drink, especially spicy or fried food, coffee and alcohol.
If your pregnancy wasn't planned then you probably won't have been taking folic acid supplement. You should start to do this now and continue taking them throughout your first trimester as it has been proven that sufficient folic acid is important in reducing the risk of spinal abnormalities in the fetus.
By the time your periods are due you are technically 4 weeks pregnant although conception only happened about 2 weeks ago. This is because pregnancy is calculated from the first day of your last period (10-14 days before ovulation) and not from conception as most people don't know the exact date that they conceived. You might experience some light spotting around this time as the fertilised egg implants in to the uterus. By the time you are 8 weeks pregnant your uterus has doubled in size, normally the size of a fist it is now the size of a large grapefruit. You may begin to feel your clothes become a bit tighter at the waist and may even need to buy a bigger bra. It might be a good idea to buy a new bra even if you don't need to go up a size yet as it is not advisable to wear under-wired bras at all during pregnancy and a good support bra that is well fitted may reduce the tendency that breasts have to sag after pregnancy.
Depending on your local hospital policy you may be given your first ultrasound scan at around this time. The midwives and consultant will use the information from the scan to confirm your dates and sometimes your EDD can change slightly. It is a great opportunity to see your baby for the first time and many women feel that their mind is put at rest and they start to worry less about complications after the first scan.