As Sherie said, its an inaccurate measure anyway, the fundal height changes with baby's position, the amount of fluid etc. You may also have a long baby but that doesn't mean big. If you are concerned about having a big baby, read New Active Birth by Janet Balaskas where you will learn things like how you can get an extra 30% space in your pelvis simply by the position you are in. Also read the article I just posted: Article: Pelvises I Have Known and Loved (about 'small pelvises')

Its nothing to fear at all - the difference between two weeks growth is so tiny, you'd be surprised how much you are worrying about.