During pregnancy the cervix is long, thick and tight as it is normally. towards the end of the pregnancy effacement happens in preparation for labour.
Effacement happens first (thinning and shortening of the cervix) this is normally when the mucus plug is lost this plug may show some blood. Effacement is measured in percentages.
Dilation is then measured in centimetres starting from 0cm (normal) to 10cm (birth)
here are some images that may explain effacement better than I can
primigravida - first time mum
multipara - mum who has already had children
