Hi Rachael,
The thing to remember with Fundal height measurements is that it is very subjective - not everyone measures from exactly the same points everytime, and so there will always be a variation in results when more than 1 mw does the measurements. The only accurate method of measuring your baby is through scanning. For this reason, with FH measurements, your personalised chart in your mat notes will probably say something about FH being measured by the same person each time. Some mw's don't do FH measurments because they are so subjective - I've only had mine done because the student mw wanted to get some practice in.
The way your baby is lying can also make a difference - for example, if baby is lying back to back, it can make the fundal height smaller, whereas lying front to back (facing your back) can make the FH bigger. You've probably noticed your bump changes shape regularly!!
One other thing that can make a difference is the amount of abdominal fat you have - I have a lot 
& so measured bigger than my dates everytime she did it anyway!!
As long as your midwife is happy, then I really wouldn't worry about it at all - if they are in any way concerned they will refer you for a growth scan or to see a consultant in ANC.
Hope this helps
Amanda
