The uterus lies deep within the pelvis between the bladder in front and the rectum behind. It is a hollow muscular organ. Before pregnancy it is the size and shape of a small pear. During pregnancy it enlarges and its position within the abdominal cavity changes. This muscle will grow in size until week 20 of pregnancy and then it will expand more and more as the baby grows.

The upper part of the uterus is known as the fundus, and is thin and contractible. On either side is a tube the fallopian tubes. The lower part of the uterus includes the cervix, which is thick and capable of tremendous distension.

During pregnancy the entrance to the cervix is sealed by a thick mucous plug which helps to protect the contents of the uterus from any infection. The cervix dips into the vagina and will open up during labour to allow the baby to move down into the birth canal and out into the world.

What happens during labour is that the top part of the uterus contracts and pulls up the lower part, stretching the cervix, which will ultimately pull up and expand until it virtually disappears. Your baby can then be born.

How the uterus is built

The muscles

The hollow uterus consists of a network of muscle fibres and bundles which run in all directions. There are three muscle layers. The muscles of the uterus contract all the way through pregnancy, but this is usually only felt by the mother in the second half of the pregnancy. These are called Braxton Hicks contractions.

The outer longitudinal layer runs up the front, over the top and down the back of the uterus. Although they cover the entire uterus, they are most numerous in the fundus of the uterus.

The middle layer consists of criss-crossing layers enclosing the many blood vessels that supply the uterus. These muscles control bleeding during the third stage of labour when the placenta is delivered.

The inner layer of circular muscle fibres makes up the cervix at the lower end of the uterus and keeps the shape of the uterine cavity. The longitudinal muscle attached to these will pull on the cervix in order to open it around the baby's head during labour.

The ligaments

Broad ligaments are so called because they cover most of the uterus, other ligaments and the ovaries. They 'blanket' the entire uterus. These ligaments support all the organs, as well as the blood vessels and nerves that supply the uterus.

Round ligaments are found radiating outwards and downwards from the top of the uterus. They lie within the broad ligament and keep the position of the uterus in a 'leaning' or 'tipping' forward position. During the pushing part of labour they contract with the uterine muscle and in this way help to prevent the uterus from 'falling out' while the mother is bearing down.

Other ligaments of the uterus act mainly as support structures for other pelvic organs such as the bladder and the rectum.

Blood and nerves

The uterus has an extremely rich blood supply in order to nourish the foetus during pregnancy. During late pregnancy up to one litre of blood passes through the uterus per minute. The nerve supply to the uterus is complex and is not yet fully understood, but what is known is that the pain receptors respond loudly to a diminished supply of oxygen and to the sensation of stretching.

It's therefore important for you to breathe through your contractions, using your own breathing patterns or learnt, altered breathing techniques.

The uterus in action

• During the first stage of labour it is the function of the uterus to thin out and open up the cervix.
• During the second stage of labour it expels the baby with voluntary effort from the mother.
• During the third stage the uterus has to contract tightly to peel the placenta off its walls and to squeeze down firmly so the amount of blood lost from the placental wound site is minimal.
• By day 14 after the birth, the uterus is back to the same size as it was before you fell pregnant!