Doctors and midwives in the UK time pregnancy from the first day of a woman's last menstrual period, not from conception. So what is called 'four weeks' pregnant' is actually about two weeks after conception. Pregnancy normally lasts for 37 to 42 weeks from the first day of your last period. The average is 40 weeks. If you're not sure about the date of your last period, then an ultrasound scan may give a good indication of when your baby will be due.

How the baby develops

In the very early weeks, the developing baby is called an embryo. Then, from about eight weeks onward, it is called a fetus, meaning 'young one'.

Week 3

(Three weeks from the first day of your last menstrual period.) The fertilised egg moves slowly along the fallopian tube towards the womb. The egg begins as one single cell. This cell divides again and again. By the time the egg reaches the womb it has become a mass of over 100 cells, called an embryo, and is still growing. Once in the womb, the embryo burrows into the womb lining. This is called implantation.

Weeks 4-5

The embryo now settles into the womb lining. The outer cells reach out like roots to link with the mother's blood supply. The inner cells form into two and then later into three layers. Each of these layers will grow to be different parts of the baby's body. One layer becomes the brain and nervous system, the skin, eyes and ears. Another layer becomes the lungs, stomach and gut. The third layer becomes the heart, blood, muscles and bones.

The fifth week is the time of the first missed period when most women are only just beginning to think they may be pregnant. Yet already the baby's nervous system is starting to develop. A groove forms in the top layer of cells. The cells fold up and round to make a hollow tube called the neural tube. This will become the baby's brain and spinal cord, so the tube has a 'head end' and a 'tail end'. Defects in this tube are the cause of spina bifida.

At the same time the heart is forming and the baby already has some of its own blood vessels. A string of these blood vessels connects baby and mother and will become the umbilical cord.

Weeks 6-7

There is now a large bulge where the heart is and a bump for the head because the brain is developing. The heart begins to beat and can be seen beating on an ultrasound scan.

Dimples on the side of the head will become the ears and there are thickenings where the eyes will be. On the body, bumps are forming which will become muscles and bones. And small swellings (called 'limb buds') show where the arms and legs are growing.

At seven weeks the embryo has grown to about 10 mm long from head to bottom. (This measurement is called the 'crown-rump length'.)

Weeks 8-9

A face is slowly forming. The eyes are more obvious and have some colour in them. There is a mouth, with a tongue. There are now the beginnings of hands and feet, with ridges where the fingers and toes will be. The major internal organs are all developing - the heart, brain, lungs, kidneys, liver and gut.

At nine weeks, the baby has grown to about 22 mm long from head to bottom.

The umbilical cord
The umbilical cord is the baby's lifeline, the link between baby and mother. Blood circulates through the cord,
carrying oxygen and food to the baby and carrying waste away again.

The placenta
The placenta is rooted to the lining of the womb and separates the baby's circulation from the mother's. In the placenta, oxygen and food from the mother's bloodstream pass across into the baby's bloodstream and are carried to the baby along the umbilical cord. Antibodies, giving resistance to infection, pass to the baby in the same way, but so too can alcohol, nicotine and other drugs.

The amniotic sac
Inside the womb the baby floats in a bag of fluid called the amniotic sac. Before or during labour the sac, or 'membranes', break and the fluid drains out. This is called the 'waters breaking'.

Weeks 10-14

Just 12 weeks after conception the fetus is fully formed. It has all its organs, muscles, limbs and bones, and its sex organs are well developed. From now on it has to grow and mature.

The baby is already moving about, but the movements cannot yet be felt.

By about 14 weeks, the heartbeat is strong and can be heard using an ultrasound detector. The heartbeat is very fast - about twice as fast as a normal adult's heartbeat.

At 14 weeks the baby is about 85 mm long from head to bottom. The pregnancy may be just beginning to show, but this varies a lot from woman to woman.

Weeks 15-22

The baby is now growing quickly. The body grows bigger so that the head and body are more in proportion and the baby doesn't look so top heavy. The face begins to look much more human and the hair is beginning to grow as well as eyebrows and eyelashes. The eyelids stay closed over the eyes.

The lines on the skin of the fingers are now formed, so the baby already has its own individual fingerprint. Finger and toenails are growing and the baby has a firm hand grip.

At about 22 weeks, the baby becomes covered in a very fine, soft hair called 'lanugo'. The purpose of this isn't known, but it is thought that it may be to keep the baby at the right temperature. The lanugo disappears before birth, though sometimes just a little is left and disappears later.

At about 16 to 22 weeks you will feel your baby move for the first time. If this is your second baby, you may feel it earlier - at about 16 to 18 weeks after conception. At first you feel a fluttering or bubbling, or a very slight shifting movement, maybe a bit like indigestion. Later you can't mistake the movements and you can even see the baby kicking about. Often you can guess which bump is a hand or a foot and so on.

Weeks 23-30

The baby is now moving about vigorously and responds to touch and to sound. A very loud noise close by may make it jump and kick. It is also swallowing small amounts of the amniotic fluid in which it is floating and passing tiny amounts of urine back into the fluid. Sometimes the baby may get hiccups and you can feel the jerk of each hiccup. The baby may also begin to follow a pattern for waking and sleeping. Very often this is a different pattern from yours so, when you go to bed at night, the baby wakes up and starts kicking.

The baby's heartbeat can now be heard through a stethoscope. Your partner may even be able to hear it by putting an ear to your abdomen, but it can be difficult to find the right place. The baby is now covered in a white, greasy substance called 'vernix'. It is thought that this may be to protect the baby's skin as it floats in the amniotic fluid. The vernix mostly disappears before the birth.

At 24 weeks, the baby is called 'viable'. This means that the baby is now thought to have a chance of survival if born. Most babies born before this time cannot live because their lungs and other vital organs are not well enough developed. The care that can now be given in neonatal units means that more and more babies born early do survive.

At around 26 weeks the baby's eyelids open for the first time. The eyes are almost always blue or dark blue. It is not until some weeks after birth that the eyes become the colour they will stay, although some babies do have brown eyes at birth. The head to bottom length at 30 weeks is about 33 cm.

Weeks 31-40

The baby is growing plumper so the skin, which was quite wrinkled before, is now smoother. Both the vernix and the lanugo begin to disappear. By about 32 weeks the baby is usually lying downwards ready for birth. Some time before birth, the head may move down into the pelvis and is said to be 'engaged', but sometimes the baby's head does not engage until labour has started.