Generally, midwife's will not measure your fundul height until you are at least 24 weeks, but mostly it starts at 28 weeks then after at every ante natal. This is because before 28 weeks, it gives really no reassurance that your uterus and baby are growing well, as some women don't have a bump at 20, 24, 26 weeks but this doesn't go to say their baby isn't ok.
This is what I was told anyway as when I went to the big hospital cos they though I was in preterm labour at almost 24 weeks, I measure 26 cm so was worried I'd have a huge baby but sure enough I went at 28 weeks and I was 28 cm. And have continued to stick on each week.