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Know your uterus

This amazing organ holds the promise of the future and will be the home of a developing human being – your baby.

The uterus is the organ which is the most affected by pregnancy. It contains and nourishes the entire conceptus (the ‘unit’ that will become the embryo, the amniotic fluid and the placenta), and will undergo massive growth and change as pregnancy progresses. The normal uterus is a small muscular organ situated in the female pelvis. In the non-pregnant condition and at the time of implantation, the uterus is pearshaped. By the third month of gestation the uterus is globular. From the seventh month to term, the uterus takes on an inverted pearshape once again. During pregnancy, as it enlarges, it gradually rises out of the pelvis, and by the fourth month of pregnancy it extends into the abdominal region. The uterus consists of a body and a cervix (neck), which have opposing functions during pregnancy. These opposing functions counter change when labour starts.

Throughout 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.

Room in the womb

The increase in size, weight and volume of the uterus during pregnancy is phenomenal. The increases in size from nonpregnant to pregnant at term are as follows:

Size: 7.5 x 5 x 2.5cm to 30 x 23 x 20cm
Weight: 50-60g to 900-1000g
Volume: 6ml to 5000ml (holding capacity of the cavity)

In the first 20 weeks of pregnancy, the increase in size is due mainly to an increase in the number of muscle fibres and also to an increase in the size of the muscle fibres. 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 stretching of the uterus causes thinning of the uterine walls and the greater the stretching, the thinner the muscle layers become, especially in the lower part of the uterus.

The upper part of the uterus is known as the fundus, and is thin and contractible. The two tubes leading off from either side are the fallopian tubes. The lower part of the uterus includes the cervix, which is thick and capable of tremendous distension.

The walls of the uterus are made up of two distinct tissue layers and a third covering layer. All three layers are suited to maintaining pregnancy and then accomplishing labour and birth. The middle layer, the myometrium, consists of smooth muscle fibres that extend in three directions. 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.

Contractions

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 (BH) contractions and are considered a normal part of a healthy pregnancy. The uterine muscle ‘tightens’ for about 30 to 60 seconds and then relaxes. Some women seem to think they never experience Braxton Hicks contractions, but actually even though you may not notice them or not recognise them, they are happening. Other women, especially those who have had a baby before, may find them rather annoying. Exactly why do they happen all the way through pregnancy?

In the early part of pregnancy, the BH contractions facilitate blood and oxygen to the placenta and the growing baby. After 30 weeks, they start to increase in intensity and are now thought to be brought about by the increase in the level of prostaglandins, and the fact that the uterine muscle becomes increasingly sensitive to oxytocin as labour approaches. These contractions prepare the uterus for labour.

Magic muscle

The uterine muscle is the only muscle in the body that has the ability to contract and retract. This means that during a contraction when the muscle shortens it will not lengthen to the same degree once the contraction has passed. This will continue until the muscle has ‘bunched up’ at the top of the uterus. Once there, it gathers its strength to contract and expel the baby. The hormone oxytocin directly causes the uterus to contract during labour and after the birth when the mother breastfeeds. The uterus has an extremely rich blood supply in order to nourish the foetus during pregnancy.

The final stages

Late pregnancy: Up to one litre of blood per minute passes through the uterus. During pregnancy the entrance to the uterus, the cervix, is sealed by a thick mucous plug, which helps to protect the contents of the uterus from any infection.

Throughout pregnancy the cervix is long and thick and tightly closed. Only when it thins out and opens up can your baby be born. On the days when BH contractions plague you and interfere with your level of comfort or your ability to go about your day, it could be your body’s way of saying ‘slow down and put your feet up.’ It could also mean that you are dehydrated and that you need to sip more water throughout the day. Your doctor may inform you that you have an ‘irritable uterus.’ If this happens be sure to listen to your body’s signals and relax.

First stage of labour: It is the function of the uterus to thin out and open up the cervix.
Second stage of labour: It expels the baby, with voluntary effort from the mother.
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 push in order for it to remain contracted. Within five days after birth your uterus will have made a rapid recovery, and by day 14 it will be the same size it was before you fell pregnant.

Fast fact
According to obstetrician and author Dr Gordon Bourne, it takes an average of 150 contractions to deliver a first baby, 75 for a second or third child, 50 for a fourth or fifth and 30 to 40 for subsequent children.

This information has been reproduced with kind permission of Zahra Publishing, publishers of Easy Health. www.easyhealth.ie.