I truly understand how worrying this is for you.

Fundal height measurements can be a bit controversial. Many practitioners use them routinely and many don't.
Fundal height is very reliant on the position of the baby. It is an indicator but not a predictor that growth may be out. It sounds like your baby is growing. That is what we want. Babies are people and we are all unique. Your little ones growth may be quite normal for her/him.
The very best indicator that your baby is doing well is movement. Of course growth is necessary but position can and will influence measurement. The other thing that can influence measurement is the amouunt of amniotic fluid surrounding your baby. Some women have an amount on the lower end of normal and others on the higher. This can and will alter the fundal height. My son was consistently measuring less than my dates would indicate. Finn was born on his due date and was 8lbs.

The good thing is that you are having this looked into. If for some reason your baby had to be born soon he/she has all the very best of odds for doing very well.

Please come back and tell us how you go. I will be thinking of you today.
Lots of love and hugs,
Deb