Dilation and effacement: how your cervix opens

At some point near the end of pregnancy, people start talking to you in centimetres — “three centimetres dilated”, “fully effaced”, “get to ten”. If you’ve nodded along without a clear picture of what’s actually measured, here’s the missing diagram in words.

Meet your cervix. It’s the neck of your uterus — through pregnancy, a firm, closed channel a few centimetres long, sealed with a mucus plug, doing the crucial job of keeping your baby in. For birth, it has to transform completely: from a closed tube into a fully open doorway. That transformation happens in two overlapping ways, and they’re the two words on everyone’s lips.

Effacement is the thinning. The cervix shortens and thins from its usual few centimetres of length to paper-thin, like a polo-neck jumper being pulled flat over a head. It’s measured in percentages: 0% is unchanged, 100% is fully thinned. With first babies, effacement usually does the bulk of its work before much dilation happens — which is why you can be “fully effaced but only one centimetre” and still be making real progress.

Dilation is the opening, measured in centimetres across: from closed (0cm) to fully dilated at 10cm — about the diameter needed for a baby’s head. The famous milestones: around 4–6cm marks the move from early labour into active labour (when contractions typically settle into a strong, regular rhythm and hospitals want you in), and 10cm is the green light for pushing. Progress is famously non-linear: the first centimetres can take days of niggling early labour, while the last few sometimes fly by in an hour.

Where the numbers come from: a vaginal examination by your midwife or doctor, feeling the cervix with two gloved fingers. It’s a snapshot, offered at intervals in labour or before things like induction — and it’s always your choice; you can accept, decline or ask to delay any examination. Worth knowing: a VE tells you where things are now, not how fast they’ll move next, which is why your team watches the whole picture (contractions, your behaviour, baby’s descent) rather than centimetres alone.

The numbers before labour mean less than you’d hope. In the final weeks it’s common to be walking around 1–2cm dilated (especially with a second baby) and stay that way for weeks — or to be fully closed at a Tuesday appointment and holding a baby by Friday. A cervix check late in pregnancy can’t predict your start date, so try not to bank on those early centimetres in either direction.

A few related terms decoded while we’re here: your mucus plug (“the show”) coming away means the cervix is changing — a sign of things warming up, not a starting gun; a stretch and sweep is a midwife encouraging this process along with a fingertip during a VE; and cervical ripening is what induction methods work on when the cervix isn’t yet soft enough to begin.

If an examination’s findings are ever relayed to you in numbers without translation — or you’d rather not know the numbers at all, which is a perfectly valid labour strategy — say so. Your midwife can tell you what it means (“things are moving; rest while you can”) rather than leaving you doing maths between contractions, and questions about your progress are always fair game.