Reassurance over smaller baby

Was just wondering if anyone had experienced this before. This is my second pregnancy, and my first daughter was born at 42w +1d at 8lb 13oz. This time round I had a growth scan last week at 27 weeks and they said my little girl is on the 10th centile based on my 'personal growth chart' which I think is based on my first daughters birth weight and my height. If she stayed on the 10th centile she would be 7lb 6oz at 40w. Because of this though need to have 2 weekly growth scans and they're concerned about her growth even though her actual growth curve is 50th centile. Are they stressing me out for no reason and should I be concerned?