Fundal measurements?

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Anyone else had a huge difference when measured? I am a size 18, now and pre-pregnancy, about 240lbs. I carry my weight everywhere pretty evenly but with baby growing, the top of my tummy is really where he is sitting. I am 24 weeks and midwife measured my fundal height at 33cm on Monday and ordered an ultrasound because of the difference. Anyone else experience this and have a normal sized baby??