Cervix Dilation and Effacement

What dilation and effacement mean

Dilation is your cervix opening; effacement is it thinning out. Both are checked during a vaginal examination, and both have to happen before your baby can be born. The US National Institute of Child Health and Human Development (NICHD) describes early labour as a time when "the cervix gradually thins out (called 'effacing') and opens (called 'dilation') to allow the baby to pass through."

Dilation is measured in centimetres. The NHS puts the end point plainly: "your cervix needs to open about 10cm for your baby to pass through it. This is what's called being fully dilated." NICE spells it dilatation; it means the same thing.

Effacement describes how thin the cervix has become, not how wide it is, and it is often given as a percentage. The Bishop score, used to assess the cervix before an induction, grades it in bands: one published version gives 0 points for 0 to 30% effacement and 3 points for more than 80%. Our Bishop score calculator shows how effacement combines with dilation, station, consistency and position.

The cervix can start changing before labour

The NICHD notes that "several weeks before labor begins, the cervix begins to soften (called 'ripening'), thin out, and open". The NHS says a show "indicates that the cervix is starting to open. Labour may quickly follow or may take a few days." More on that in our page on the mucus plug and bloody show.

Effacement vs dilation: which comes first?

None of the NHS, NICE or the American College of Obstetricians and Gynecologists (ACOG) sets a fixed order. They describe both as parts of the same early change. NICE defines the latent first stage of labour as a period, "not necessarily continuous", with contractions and "some cervical change, including cervical position, consistency, effacement and dilatation up to 4 cm." The NHS calls it the time when "your cervix becomes soft and thin, and starts opening". ACOG defines the onset of labour as regular, painful contractions "resulting in cervical dilation or effacement or both."

That matters. A cervix that has moved forward, softened and thinned has made real progress even if the centimetre figure has barely changed. Our page on the latent phase of labour covers what that stage is like to live through, and prodromal labour covers the stop-start version.

Dilation and the stages of labour

The centimetre that marks "proper" labour depends on which guideline your unit follows. In the UK, NICE counts established labour from 4 cm. In the US, ACOG's 2024 labour guideline "recommends that cervical dilation of 6 cm be considered the start of the active phase of labor."

Why the gap? ACOG explains that the 4 cm marker comes from Emanuel Friedman's labour curves of the 1950s. The Consortium on Safe Labor, which analysed 62,415 births in 19 US hospitals and was published in 2010, found that dilation speeds up after about 6 cm and that progress from 4 to 6 cm was "far slower than previously described". So a 5 cm cervix is established labour in UK notes and may still be latent labour in a US hospital. Our page on the stages of labour covers each stage in full.

Why progress is not a straight line

In the Consortium on Safe Labor study, "labor may take more than 6 hours to progress from 4 to 5 cm and more than 3 hours to progress from 5 to 6 cm of dilation." First and later labours moved at a similar pace before 6 cm; after it, later labours sped up much faster.

The old rule of a centimetre an hour comes from Friedman, whose data put the slowest normal active-phase rate (the 95th percentile) at 1.2 cm an hour in a first labour. ACOG now gives that figure as 0.5 to 1.3 cm an hour, and defines a slow, or protracted, active phase as less than 1 cm in 2 hours.

NICE uses a different yardstick: it asks clinicians to consider delay in established labour if dilatation is less than 2 cm in 4 hours (or slowing, in a second or later labour), weighed alongside the descent and rotation of your baby's head and changes in your contractions. A slow spell starts a conversation; it is not automatically a reason to intervene. See when labour stalls.

How dilation and effacement are checked

By a vaginal examination, which is an offer you can accept or decline. NICE lists six findings: how far down your baby is (station), its position, any swelling or moulding of the head, effacement, dilatation, and whether the membranes are intact. In established labour NICE asks teams to offer an examination every 4 hours, or in response to your wishes if there is concern about progress. The NHS: "If you do not want to have these, you do not have to." See our guide to vaginal examinations in labour.

There is no reliable self-check. ACOG says that sometimes "the only way to tell the difference" between false and true labour "is by having a vaginal exam to find changes in your cervix that signal the start of labor."

When to call

You act on contractions and symptoms, not centimetres. The NHS asks you to call your midwife or maternity unit for guidance if you think you are in labour, or you are having regular contractions every 5 minutes or more often.

Call urgently if your waters break, you have vaginal bleeding, your baby is moving less than usual, you are less than 37 weeks pregnant and think you might be in labour, any contraction lasts longer than 2 minutes, or you are having 6 or more contractions every 10 minutes. Call 999 if you think your baby is coming now and you have a strong urge to push.

In the US, ACOG says to go to the hospital if your water has broken and you are not having contractions, you are bleeding heavily from the vagina, you have constant, severe pain with no relief between contractions, or the baby is moving less often.

Frequently asked questions

What does effacement mean in pregnancy?

Effacement is the thinning of your cervix, the neck of the womb, as labour approaches and begins. It is separate from dilation, which is the cervix opening, and is often recorded as a percentage. NICE counts effacement among the cervical changes of the latent phase, which comes before established labour at 4 cm.

Can you feel your cervix dilating?

Not as a sensation guidance describes. What you notice are the signs the NHS lists: contractions or tightenings, a show, backache, an urge to open your bowels, and your waters breaking. Dilation itself is confirmed by a vaginal examination, and ACOG says that is sometimes the only way to tell false labour from true.

What is a cervix dilation chart with fingers?

Charts that match centimetres to finger widths circulate online, but NICE, the NHS and ACOG all describe dilation in centimetres, assessed during a vaginal examination. Treat finger charts as a rough picture of size, not a way to judge your own labour.

What is station, and how does it relate to effacement?

Station is how far down your baby has moved into your pelvis. NICE lists it alongside effacement and dilatation as findings to note at a vaginal examination, and the Bishop score adds station, effacement and dilation to the cervix's consistency and position to judge how ready it is before an induction.