I know fundal height (the length of the fundus, whatever that is, from the pelvic bone) tends to match your weeks. So, at 18 weeks, fundus length is 18cm, and at 20 weeks it's 20cm. Which puts the baby's size at a bit smaller to fit inside.

But I was measuring 20cm at 19 weeks, and don't babies grow in growth spurts from the 2nd trimester onwards anyway, so the baby might be small for dates one week, and then much bigger the next..I know some places don't even measure so it can't be that important.