Femur measurements 1 week behind?!?

Audrey • I'm a first time mum and baby will be due end of July! My husband and i are very very excited!
The last 2 visits to the OB i realised my baby's femur measurements are behind "schedule". My EDD is 31st July but femur measurements is about 5th-7th August. Today, it showed 28th percentile! I can't help but worry :(
Can anybody shed some lights on this? 
Thanks a lot!