The uterus is the organ most affected by pregnancy. It contains and nourishes the entire conceptus (the unit that will become the embryo, amniotic fluid and placenta), and undergoes massive growth and change as pregnancy progresses.
How the uterus changes shape during pregnancy
The normaluterus is a small muscular organ situated in the female pelvis. Normally, the uterus is pear shaped. By the third month of gestation the uterus is globular. From the seventh month to term the uterus takes on an inverted pear shape once again. As it enlarges during pregnancy it gradually rises out of the pelvis and by the fourth month extends into the abdominal region.
The uterus and cervix during labour
The uterus consists of a body and a cervix (neck). During pregnancy the body of the uterus must remain relaxed and stretch in order to accommodate the foetus and placenta while the cervix must remain firm in order to maintain the pregnancy.
Once labour starts these functions are reversed: the body of the uterus must contract and retract while the cervix must relax, stretch and open to allow the baby to pass through.
The size and weight of the uterus during pregnancy
The increase in size, weight and volume of the uterus during pregnancy is as follows:
- Size: 7.5 x 5 x 2.5cm to 30 x 23 x 20cm
- Weight: 50g to 1kg
- Volume: 6ml to 5 litres (holding capacity of the cavity
How the uterus increases in size
In the first 20 weeks of pregnancy, the increase in size is due mainly to an increase in the number if muscle fibres, as well as their size. In the second half of pregnancy the increase in size is due mainly to the mechanical stretching of the uterus by the developing and growing conceptus, which has filled the cavity by the 12th week.
The uterus contracts painlessly all the way through pregnancy, but these contractions are usually only felt by the mother during the second half of the pregnancy. In the early stages of pregnancy – long before you are aware of them – they are forming what is known as the ‘lower uterine segment’. This area is adjacent to the cervix and softens and stretches during early pregnancy, to accommodate the growing conceptus.
During early pregnancy the contractions are mild and irregular, but although they remain painless, they will gradually increase in frequency and intensity as the pregnancy nears term. These are known as Braxton-Hicks contractions and are a normal part of a healthy pregnancy, preparing the uterus for labour.
The role of oxytocin
The uterine muscle is the only muscle in the body that has the ability to contract and retract. The hormone oxytocin directly causes the uterus to contract during labour and after the birth when the mother breastfeeds.
Nourishing the foetus during pregnancy
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 per minute passes through the uterus.
How the uterus is protected from infection
During pregnancy the entrance to the uterus, the cervix, is sealed by a thick mucus plug which helps to protect the contents of the uterus from any infection. During pregnancy the cervix is long and thick and tightly closed.
The function of the uterus during labour
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 of labour the uterus has to contract tightly to pop the placenta off its walls and to squeeze down firmly so the amount of blood loss from the placental wound site is very little.
As the uterus continues to contract and retract the many blood vessels are ‘pinched’ off and bleeding stops. The uterus will be “rubbed up” by your midwife or doctor and you will be asked to continue to do so in order for it to remain contracted. Within 5 days after birth your uterus has made a rapid recovery and is back where it started and by day 14 it is the same size as it was before you fell pregnant.