She might just be a smaller baby. My bump started to measure small at 35 weeks so I was sent for some ultrasounds to make sure the baby was growing okay. I had Oliver 9 days late and he weighed 6lb 3oz, tiny but completely healthy! Just a small baby. He's completely made up for it now though.
Growth scans can also be a bit misleading. At my 2nd growth scan, when I was 38 weeks, I was told Oliver was estimated to be 7lb 3oz, then I gave birth 3 weeks later to a baby at 6lb 3oz so they got it completely wrong! My SIL was told she was going to have an underweight baby from scans then gave birth to a 8lb 7oz baby so it's not always accurate. xx