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Anatomy of pregnancy and birth - cervix

4-minute read

The cervix plays a critical role throughout a woman’s life, but never more so than during conception, pregnancy and childbirth. The cervix changes during these events, so it’s important to understand the role of this organ and what these changes mean for you and your baby.

Where is the cervix?

The cervix is the muscular channel or opening that runs between the uterus (also known as the womb) and the vagina. The cervix is located in the pelvis and lies behind the bladder and near the front of the rectum. It is held in its position by tough, flexible ligaments.

What does the cervix do in women who are not pregnant?

In women who are not pregnant, the cervix is narrow and firm. Its size, shape and lining undergo subtle changes throughout a woman’s menstrual cycle.

For instance, the cervix has a smooth covering of mucus. When a woman is ovulating, the mucus becomes stretchy and elastic, which helps sperm move and survive following intercourse, and is known as ‘fertile mucus’. When a woman is not ovulating, the mucus takes on a more creamy texture and can be milky or yellow in colour. This change in consistency indicates that sperm are less likely to reach the uterus after intercourse at this time in the menstrual cycle.

The cervix contains a channel, referred to as the 'cervical canal', which also has an important role in women who are not pregnant. Sperm enters the uterus after intercourse and menstrual blood leaves the uterus during menstruation through this channel.

What is the function of the cervix during conception and pregnancy?

The cervix has several important functions during conception. After a woman conceives, the mucus on the surface of the cervix becomes sticky and thick. This stops new sperm and microorganisms, such as bacteria and viruses, from entering the uterus and potentially harming the fetus.

The cervix also plays a crucial role during pregnancy. From conception until just before birth, the cervix protects the growing baby by increasing in size and strength, helping to keep the baby safe and secure in the uterus.

How does the cervix change during labour?

The cervix undergoes dramatic changes during labour in a process known as ‘effacement’ or ‘ripening’. It starts out long and firm, and as labour progresses, it softens, widens and becomes more elastic.

Changes to the cervix are partly triggered by contractions of the muscles of the uterus, which help open the cervix. As the cervix begins to open, you may see a clear, pink, or slightly bloody discharge (sometimes referred to as the 'show'). This is the release of the mucus plug, that sits over the cervical canal during pregnancy.

The first stage of labour begins when you feel contractions — your cervix will start to soften and to open. The time it takes for the cervix to open (or dilate) will vary from woman to woman and can range from several hours to days or even weeks. Once the cervix is dilated more than 3cm, and contractions are strong and regular, labour is said to be ‘established’.

Established labour is also known as the 'active phase'. This phase continues until the cervix dilates to approximately 7cm, with the contractions being strong, painful and around every 3 or 4 minutes.

This is followed by the ‘transition phase’, when the cervix continues to fully open. There is now less time to rest in between contractions.

First stage of labour is complete when the cervix is fully open (referred to as 'fully dilated'), the baby moves through the pelvis and the mother can begin pushing the baby out. Being fully dilated might not actually be a 10cm opening of the cervix. Rather it is when the doctor or midwife cannot feel any cervix when they perform a vaginal examination.

The second stage of labour is when your baby is being born and the third stage is when the placenta is delivered.

What problems can occur with the cervix during pregnancy, labour, or birth?

The cervix is monitored throughout pregnancy, labour and birth to identify any problems. For instance, if the cervix is weak, it might be difficult to keep the baby in the uterus for the whole pregnancy. This can lead to premature labour and birth.

During labour, contractions might not be regular or strong enough to fully open the cervix, which slows labour down. In these situations, the doctor or midwife might intervene by breaking the mother's waters or using a hormone to increase contractions and speed up the dilation of the cervix.

It is important that the mother does not begin pushing the baby out until the cervix is fully dilated. This is because pushing too soon might swell the cervix, which can slow down the birth.

Read more here about complications in labour, and how they are managed.

Learn more here about the development and quality assurance of healthdirect content.

Last reviewed: October 2020

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