The cervix in labour

Most people are aware that for a baby to get out the cervix has to open. One of the ways of monitoring progress in labour in most maternity systems is through performing vaginal examinations (VE). These are offered to see how open the cervix is, also referred to as dilation. These examinations also give other information such as how low the baby is in the pelvis and the position the presenting part of the baby is in (usually head, sometimes bottom!) The concept of the cervix opening is somewhat misleading, and results in a lot of confusion. Especially when told that the cervix opens to 10cm… don’t worry no one is going near your cervix with a ruler.

So, let’s break this down.

The cervix is 3-4cm long and ‘closed’ before labour. The os is the small hole that runs through the centre of the cervix. During pregnancy there is a mucous plug in the cervix to protect the inside of the uterus and the baby from anything that could travel up inside (for eg. bacteria that could cause an infection). The cervix is literally what keeps the baby in during the pregnancy – it is mighty strong!

When your body begins to prepare for going into labour the cervix will begin a process of changing, referred to as ‘ripening’ – as if like fruit. The cervix will become shorter and softer (known as effacement), and begin to open (dilate). It will also start to move round from a posterior position, pointing towards your tailbone, towards a more central and eventually forward facing position. The cervix begins to open as a result of hormonal changes and also with the weight of the baby as they enter the pelvis (referred to as ‘engagement’), applying direct pressure to the softening cervix.

The cervix opens further when contractions start. Contractions are the tightening of the uterine muscles that help to draw the cervix up into the body of the uterus and help move the baby downwards. So when it is said that a woman is ‘10cm dilated’ or ‘fully dilated’ it really just means that the cervix can no longer be felt as it has been drawn completely up around the side of the baby’s head. Imagine a roll-neck jumper: before you pull it on the neck is a smaller hole and the material is thicker, as it stretches over your head the material becomes thinner and the hole opens. This is what the cervix does! Below are some pictures to help visualise this… use you imagination a bit as the ‘uterus’ is knitted and technically upside down! But hopefully what happens to the cervix makes sense.

And with a pelvis involved…

Note: For the purposes of showing the ‘cervix’ in these pictures the pelvis is resting flat, which would actually mean the woman would be lying on her back. This is not a helpful position for labour.

I hope that this has stripped away some of the mystery from what the cervix does in labour and how it opens. The million pound question is: ‘how long does labour take?’ and that will have to wait for another blog post!

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