Im not quite as far along as you yet lovely, but ive come to realise that most books and internet sites all have different sizes. Im 24 weeks and according to My preg book she should be 28 cm, some say 30cm, and when i googled it it came up with 34cm (no way is she tht long yet!). At my 20 week scan she was about 25cm I think. If your MW says everythings fine then i shouldnt worry too much...theres so much info out there that half the time im sure most of its wrong!! Everyones different, so im sure everything for you its perfectly normal! 
xxx
