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How Your Cervix Dilates And Why It’s Unpredictable

During the first stage of labour, what the cervix is doing, or not doing, gets plenty of attention.

This focus can put a lot of pressure on women.

They might feel as though it’s a question of doing well, or failing in some way, depending on how dilated they are.

Cervical checks are a common, but not completely necessary, part of maternity care in most hospital settings around the world.

What your dilation is every 3-4 hours has the potential to determine your birth outcome.

But is cervical dilation a good predictor of how your labour will unfold?

What Is The Cervix?

The uterus is a muscular organ and is shaped like an upside-down pear. The cervix is actually part of the uterus, but it’s made of different tissue.

There are three parts to the uterus:

  • the fundus (the dome-shaped top of the uterus)
  • the body (the larger central part), and;
  • the cervix (the opening into the vagina).

The cervix extends like a canal from the uterus into the top of the vagina.

The cervical opening into the uterus is called the internal os, and the opening into the vagina is called the external os.

How Does The Cervix Dilate?

During pregnancy, your cervix is thick and closed.

It is filled with a mucus plug that prevents bacteria travelling into the uterus and causing infection.

In the weeks leading up to labour, hormones begin to act on the cervix, causing it to soften, thin and open slightly. This can vary from woman to woman, and from pregnancy to pregnancy.

In some women the cervix might be completely closed until labour begins. Others might experience pre-labour contractions, which dilate the cervix to 3-4 centimetres (or even more) before labour starts.

Most of the work of labour is done in the first stage, when contractions are opening your cervix.

The uterus is made up of layers of muscles; some go around the uterus and others go from the top to bottom.

As these muscles contract, the cervix begins to thin.

This means the lower part of the cervix, which extends into the vagina, begins to shorten and pull up so it becomes part of the lower uterus.

Once the cervix has softened and thinned, it will open, or dilate, to allow your baby to be born.

Find out more about cervical dilation.

Is Cervical Dilation Predictable?

The dilation stage of labour is usually the longest. If you give birth in a hospital setting, this is the stage that tends to have the most time restrictions placed on it.

This is because of a long held belief that there is a normal process of dilation – a straight line beginning from 0cm and increasing in 1cm increments, every hour, until you reach 10cm, or full dilation.

The basis for this belief is a study done over 60 years ago, which resulted in a graph called Friedman’s Curve.

Despite current research showing Friedman’s Curve to be irrelevant, some maternity care policy is still based on this outdated information.

Read more about how Friedman’s Curve is used in labour.

Recently more research has emerged to confirm that cervical dilation is not as predictable as has been asserted in the past. The study looked at women who went into labour spontaneously, and progressed on their own.

The results showed large variations, both before and after 6cm dilation (the beginning of active labour).

What Makes Cervical Dilation Variable?

The speed of cervical dilation depends on three things: the individual woman, her baby, and the environment she is labouring in.

If you are in an environment where you feel safe and supported, and if you and your baby are healthy and not at risk for complications, it’s likely your cervix will dilate without too many variations.

You might not dilate 1cm per hour, but dilation will most likely increase, rather than stall or stop.

This scenario is rarely played out in hospital environments. Often the process of labour is interrupted many times. This can cause cervical dilation to slow, stall, or even reverse.

These interruptions can begin with the journey into hospital, and continue during the admission process, and during shift changes, or vaginal examinations. Constant changes are invasive, and interfere with the production of oxytocin, the hormone responsible for contractions.

Read more about how vaginal examinations can interfere with your labour hormones.

During the first stage of labour, if your body begins to release stress hormones, it’s a signal things aren’t safe enough to give birth, and the process of labour is stalled or reversed. This is an inbuilt security system all mammals have, to ensure their young are born safely.

Find out more about why aiming for an undisturbed labour has many benefits.

The environment and a woman’s personal sense of safety play an important role in how labour progresses. Maternity care in hospitals, however, tends to be the exact opposite of what is needed to promote effective labour and positive birth outcomes. Care providers should be aware of current research regarding cervical dilation, and how variable it can be, depending on the individual woman.

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