I think they watch it from a blood pressure point of view? Possibly for pre-eclampsia monitoring. I think the fact that you are monitored more closer can only ever be a good thing, and it isn't necessarily a sign of something wrong - just being careful. You could call the your midwife dept though, and just explain your concerns / have a chat. I did this after one of my scans for a different reason and they were really lovely, and very happy to calm my fears and provide explanation.

Hi am 22 weeks pregnant with 1st baby and just had my anomaly scan. Everything was fine and baby is growing well, I haven't got any medical conditions and everything's been going smoothly. I was told that I have an increased placental blood flow to the baby and have been told to come back at 28 and 36 weeks for further scans to check growth.
In a bit confused as to what this means and wondered if anybody's had the same and what the outcome was? Or if any one knows a bit about it and can give me a bit of info.
Thanks so much.