Weeks 1-4
Fertilisation occurs, then a quickly multiplying mass of cells embeds itself into the lining of the uterus. Gestational age in 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.

Week 5
The mass of cells is developing fast and becomes an embryo. For many women the first sign of pregnancy is a missed period. Home Pregnancy tests are considered largely reliable but if a first test is negative, a second one a few days later may prove positive, as hormone levels in the urine rise.

Week 6
The embryo officially becomes a fetus. It is about the size of a baked bean and its spine and nervous system begin to form. The fetus already has its own blood system and may be a different blood group from it's mother. Blood vessels are forming in what will become the umbilical cord and tiny buds which will become limbs appear.

Week 7
The baby's heart is beginning to develop. Morning sickness and other side effects of early pregnancy may take hold. Around this time many women find they experience the side-effects of early pregnancy including needing to urinate more often, nausea, vomiting, and feeling a bit weepy or irritable. All medication including supplements need to be carefully checked as the fetus is undergoing vital development in the first 12 weeks.

Week 8
It is quite common to have a first scan at this stage if the woman has had a previous miscarriage or bleeding. An early scan is often done through the vagina and is used to check the pregnancy is not ectopic. It should show the baby's heartbeat. The nervous system is also developing rapidly, especially the brain. The head gets bigger and the eyes form under the skin of the face. The fetus's limbs are growing and look more like arms and legs. All internal organs are developing and becoming more complex.

Week 9
The fetus is about 5cm long with its head tucked onto its chest. It has most its major organs and eyes and ears are developing.

Week 10
A scan at 10-13 weeks is recommended to pin down the date of the pregnancy, and to check on the baby.

Week 11
The umbilical cord is fully formed providing nourishment and removing waste products. The fetus looks fully human now.

Week 12
By this week the threat of miscarriage is much reduced. This is the point when many women announce their pregnancy to friends and family. The fetus is growing in length much more quickly by now it is about eight cm long and weighs about 2 oz. The placenta is now 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.

Week 13
The womans uterus is becoming larger and is starting to rise out of the pelvis. The fetus can move its head quite easily.

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

Week 15
Screening for Downs syndrome is offered about now. 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. These diagnostic tests have a small risk of subsequent miscarriage.

Week 16
The fetus now has toe and finger nails, eyebrows, and eyelashes. It is also covered with downy hair. The hair that will cover the baby until the last week or so of pregnancy - called lanugo - starts to form. This hair is very fine more like down and it probablys likely serves as some form of insulation and protection for the skin.

Week 17
The fetus can hear noises from the outside world. By this stage the mother is visibly pregnant and the uterus is rising.

Week 18
By this stage the fetus is moving around a lot - probably enough to be felt.

Week 19
The fetus is now about 15-20cm long and weighs about 300g. Milk teeth have formed in the gums.

week 20
Half way through pregnancy now. Almost all mothers are offered a routine scan. The foetus develops a waxy coating called vernix. 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.

Week 21
The mother may feel short of breath as her uterus pushes against her diaphragm leaving less space for the lungs. The mother may be offered another ultrasound scan around this time. The scan can check the baby´s spine internal organs and growth are normal.

Week 22
Senses develop: taste buds have started to form on the tongue and the foetus starts to feel touch.

Week 23
The skeleton continues to develop and bones that form the skull begin to harden - but not fully.

Week 24
Antenatal checkup and scan to check the baby´s position. A baby born this early does sometimes survive. A baby born at 24 weeks may possibly survive but it would have severe breathing difficulties as its lungs would not be strong enough to cope. It would also be very thin lightweight and susceptible to infections.

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.

Week 26
The foetus skin is gradually becoming more opaque than transparent.

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

Week 28
Routine checkup to 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.

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.

Week 30
Braxton Hicks contractions may begin around now. They are known as 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.

Week 31
The fetus 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.

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

Week 34
The mother may find it more difficult to eat full meals as the expanded uterus presses on her stomach.

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

Week 36
The baby´s head may engage in the pelvis any time now.

Week 37
The baby´s lungs are practically mature now and it can survive unaided. The final weeks in the womb are to put on weight.

Week 38
Babies born from this week onward are not considered early.

Week 39
Another ante-natal appointment. The mother has reached her full size and weight by now.

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

Week 41

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.