They feel your bump to check where the uterus has grown up to, and as you get further along in your pregnancy it's used to check the position of the baby ie. head down etc.
Feeling the bump won't determine how many cm it's measuring, so thats why they measure it.
I started getting measured at 12 weeks; I was 12cm. At 16 weeks, I was 17cm. They say you should be approximately the same # of centimeters as weeks. It's the measurement from your public bone to the top of your uterus.
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