Femur length growth scan


Hi, just had my 28 week growth scan and for some reason my twin 2 femur length was below 5th percentile whereas everything else such as head and abdominal circumstance was up in the 70 and 80%! Is this something to worry about as its the first time its come up on a scan...i feel very anxious after i googled it as it mentioned genetic disorders. Could this be just due to them being twins and not much room?