LOL, DO the maths, C=pi d - 10cm dilation = 31.1418cm head. 11cm dilation = 34.559cm head. 12cm dilation = 37.701cm head. And 35cm is a smallish head - my friend delivered a 9lb boy with a 39cm head - are we going to say there's no real difference between 10cm and 12.5? When the difference between 2.5cm and 5cm is how one is diagnosed as in labour or not, and a woman who dilates to 7.5 then stops isn't "full" she's "failure to progress". Based on the maths the MAJORITY of women are aiming for at least 11cm, probably more.
Full dilation is when the cervix has been fully drawn up and there is no longer any distinction to feel between the vagina and the uterus, it's one smooth tunnel. Obviously not all women would be exactly the same number of cm across, even if babies heads and pelvic outlets were completely round, which they aren't.
The just call full dilation 10cm, even if it's more like 8cm or 13cm. Dilation at NO stage indicates anything. Some women are at 4cm dilated for the last 5 weeks of pregnancy, others go from 2-"10" in under 30mins. It's a poor clinical signpost to rely on, it tells you nothing about how close labour or birth is and it is a source of worry for the mother, worry which will ironically close the cervix and leave her feeling defeated if after hours of labour she is "only" a certain number of cm.
Dilation is worth checking if a woman is pushing with no results for more than 40 minutes or so (possible anterior lip), or if the baby seems very high still when mum wants to push (possible posterior position) but for a normal woman in a normal pregnancy or labour it is unnecessary and unhelpful. I went into transition at 3cm, my transition was TEXTBOOK, my cervix was not. I delivered an hour later.
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