Page last updated at 17:06 GMT, Wednesday, 2 September 2009 18:06 UK

Pregnancy timeline

A timeline of some of the key changes to mother and baby during pregnancy.

Image of foetus at 10 weeks
Foetus at 10 weeks

In the UK pregnancy is calculated from the first day of the woman's last period so for as much as three weeks of this first month she might not be actually pregnant.

The fertilized egg begins as a single cell which quickly multiplies to form an embryo as it travels towards the womb. The embryo attaches itself to the womb lining, which is already thickening to support it. For many women the first sign of pregnancy is a missed period in week five.

Shop-bought tests are considered largely reliable so the mother-to-be does not have to have her pregnancy confirmed by her GP. If a first test is negative a second one a few days later may prove positive as hormone levels in the urine rise.

The embryo is now about the size of a baked bean and its spine and nervous system begin to form.

It already has its own blood system and may be a different blood group from its mother. Blood vessels are forming in what will become the umbilical cord and tiny buds which will become limbs

The baby´s heart is beginning to develop by week seven. Around this time many women find they experience the side-effects of early pregnancy including needing to urinate more often nausea and vomiting and feeling a bit weepy and irritable.

All medication including supplements need to be carefully checked as the baby is undergoing vital development in the first 12 weeks. If the woman has not told her GP or community midwife she is pregnant yet, now is a good time to do so.

It is quite common to have a first scan at week eight if the woman has had a previous miscarriage or bleeding. At about this time the embryo officially becomes a foetus.

An early scan is often done through the vagina and is used to check the pregnancy is not ectopic. It should show up the baby´s heartbeat. The nervous system is also developing rapidly especially the brain. The head gets bigger and eyes form under the skin of the face. The foetus' limbs are growing and look more like arms and legs. All internal organs are developing and becoming more complex.

Women who may be at higher risk of passing on sickle cell anaemia and thalassaemia should be offered a blood test before 10 weeks.

These conditions are rare but serious inherited blood disorders. They are more common in people of certain ethnic backgrounds including African, Caribbean and some Asian and Mediterranean communities. If the mother is found to carry the gene for either condition, the father is offered a test. If both parents are found to be carriers then they are offered the option of testing the foetus in the womb.

WEEK 10-19
Foetus at 14 weeks
Foetus at 14 weeks

A scan at 10-13 weeks is recommended to estimate when the baby is due and to check whether there is more than one baby. This scan may also be part of a screening test for Down's syndrome.

Testing for Down's Syndrome can take place between now and 13 weeks. The nuchal translucency scan is used to measure the amount of fluid under the skin at the back of the baby's neck. If it is increased this raises the risk of Down's syndrome. A blood test is taken at the same time which measures two hormones in the blood. The combined results of both tests will be used to give the mother a statistical guide to her chances of having a baby with Down's Syndrome.

The umbilical cord is fully formed providing nourishment and removing waste products. The foetus looks fully human now.

By week 12 the threat of miscarriage is much reduced. By now the foetus is fully formed and many women announce their pregnancy to friends and colleagues.

The foetus is growing in length much more quickly, by now it is about eight cm long and weighs about 60 grams. The placenta is also well formed though it's not yet doing its full job it takes over fully in week 14. The mother is likely to have her first scan this week.

The woman's uterus is becoming larger and is starting to rise out of the pelvis and the pregnancy is probably beginning to show. The foetus can move its head quite easily although the mother may not feel any movement yet.

Week 14 is one third of the way through. The average pregnancy lasts 266 days or 280 days from the first day of last period.

If screening for Downs syndrome wasn't carried out earlier, it is offered about week 15. A simple blood test is carried out first then further tests may be offered.

On the basis of the blood test results the woman may opt for a Chorionic Villus sample or an amniocentesis which would diagnose Down's syndrome or other chromosomal abnormalities. However these diagnostic tests have a small risk of subsequent miscarriage. An alternative to blood tests is a nuchal translucency scan a new scan offered by some larger hospitals. But again an amniocentisis would be required for firm diagnosis.

The foetus has toe and finger nails eyebrows and eyelashes by week 16. It is also covered with very fine, downy hair, called lanugo. This hair probably serves as some form of insulation and protection for the skin and it disappears before birth or shortly after.

Most women who've been suffering from morning sickness are usually feeling better by now.

In week 17 the foetus can hear noises from the outside world. By this stage the mother is visibly pregnant and the uterus is rising.

Ultrasound scan or anomaly scan to check the physical development of the baby, is made during weeks 18-20. The scan can show the foetus in fine detail and often reveal if the baby is a boy or a girl. However not all hospitals offer to tell parents the sex of the child - and not all parents want to know.

By this stage the foetus is moving around a lot - probably enough to be felt.

At week 19 the foetus is now about 15-20cm long and weighs about 300g. Milk teeth have formed in the gums.

WEEKS 20-29
Foetus at 20 weeks
Foetus at 20 weeks

Half way through pregnancy now. Some women experience a surge of energy. The foetus develops a waxy coating called vernix, which is probably provides protection for the skin as the baby floats in amniotic fluid.

The mother's uterus begins to push against her diaphragm leaving less space for the lungs. Feeling slightly short of breath can be normal at this stage, but the mother should seek medical advice if she experiences these symptoms.

The foetus's senses begin to develop at about week 22: taste buds have started to form on the tongue and the foetus starts to feel touch.

The skeleton continues to develop and bones that form the skull begin to harden - but not fully.

In week 24, an antenatal checkup and scan will check the baby´s position. A baby born this early can now survive with medical help, although it may be prone to breathing difficulties as its lungs would not be strong enough to cope. It would also be very thin lightweight and susceptible to infections.

At week 25 all organs are now in place and the rest of the pregnancy is for growth. Preeclampsia is a risk from here onwards.

This potentially fatal condition causes high blood pressure protein in the urine and swelling caused by fluid retention. The causes are unclear but research suggests it may be linked to an immune reaction to the foetus or the placenta. If the condition is serious women may be advised to take drugs to lower their blood pressure and in some cases an early caesarean or induction may be performed. Serious complications of pregnancy

The foetus skin is gradually becoming more opaque than transparent.

The foetus measures about 34cm and weighs about 800g by week 27.

In week 28 a routine checkup will test for preeclampsia. Women with Rhesus negative blood will also be tested for antibodies.

If the mother has Rh negative blood but the baby is Rh positive she can develop antibodies to her baby´s blood during labour. This is not a problem in the first birth but can affect subsequent pregnancies and result in stillbirth. Fortunately treatment is simple and effective. BBC Health: Ask the doctor - Rhesus disease

In week 29, some women develop restless leg syndrome in their third trimester.

This is sensations such as crawling tingling or even cramps and burning inside the foot or leg - often in the evening and at night disturbing sleep and making the mother feel she needs to get up and walk around. No-one knows what causes this harmless but irritating condition.

Mum and newborn baby
Mother and newborn baby

Braxton Hicks contractions may begin around now. They are practice contractions which don't usually hurt.

These are irregular, painless contractions which feel like a squeezing sensation near the top of the uterus. If contractions become painful or occur four times an hour or more, the woman should call a doctor as she may be in early labour.

By week 31 the foetus can see now and tell light from dark. The mother's breasts start to produce colostrum about now

This high calorie milk is produced by the mother to feed the baby for the first few days after birth before normal milk starts.

Week 32 - another antenatal appointment. The foetus is about 42cm and weighs 2.2kg. A baby born now has a good chance of survival.

From week 33 the baby should become settled in a head downwards position. A midwife can help to move it if necessary.

The mother may find it more difficult to eat full meals as the expanded uterus presses on her stomach.

If the mother has been told she may need a planned caesarean, week 35 is probably a good time to discuss it further.

From about week 36 the baby´s head may engage in the pelvis any time now.

Any baby born after week 37 is considered to be full term.

The baby´s lungs are practically mature now and it can survive unaided. The final weeks in the womb are to put on weight.

Another ante-natal appointment. By week 39 the mother has reached her full size and weight by now.

In theory the baby should be born in week 40. The mother´s cervix prepares for the birth by softening.

First babies are often up to a week late but if there are signs of distress to mother or child the birth will be induced.

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