We are building a week by week guide to pregnancy.  It is not yet finished but the weeks that are complete are listed below.

(Editors note:  I have used the convention of describing the future baby as female so I apologise to all those mums and dads who know that their future offspring will be a little boy - it must be really annoying. Sorry.)

The First Trimester (Week 1 to12)

Baby will grow and develop quickly in the first trimester. Most of the organs, including the nervous system and heart, are formed by the time you are nine weeks pregnant (seven weeks after fertilisation). At this point the embryo is known as a foetus.

The placenta is fully formed by the time you are around 10 weeks pregnant (eight weeks after fertilisation). Nutrients are transferred from you to baby through the placenta. Waste products from the baby are returned to your circulation to be removed.

By the time you are 12 weeks pregnant (10 weeks after fertilisation), your baby is approximately 60mm long. At this stage your baby's neck is uncurling and the limbs are complete. The eyelids are still fused but ears are forming.

The Second Trimester  (Week 13 to 26)

In the second trimester baby's sex organs develop and other organs mature. The baby swallows amniotic fluid and passes it out through its gut. The kidneys start to work and pass small amounts of urine.

Your baby can now hear, and is covered in fine hair called lanugo. By the end of the second trimester babies would have a chance of surviving she was born, but would need intensive care.

The Third Trimester  (Week 27 to 40 hopefully)

Baby's lungs mature throughout the third trimester. The baby makes breathing movements, even though the lungs don't work properly until birth. Fat stores are laid down in preparation for birth. The baby grows fine hair and fingernails, the eyes open and close, and teeth may start growing under the gums.