Advice

Hi, Im 36 +3 days and recently had ultrasound when I was 35+6 weeks. My baby was measuring all lovely and was all healthy except for the femurs where they was measuring for 29 weeks. Have anyone else this happened? I'm a first time mam, and no idea if this means anything? I've got to go down in 1 week for another scan, to see if there was any growth in femur. The midwife said it was nothing there he worry about but I've done a bit of research and come across it could mean a sings of DS. I can't help but worry. Please someone help with advice!