Head Circumference/Femur in Low Percentile

I had a 21 week anatomic ultrasound and have been told the baby's head circumference and femurs are in a very low percentile. Everything else seems to be developing normally, including his brain. The doc proceeded to launch into a conversation stating we need to watch the grow to check for development to possible genetic/congenital disorders to options for termination of pregnancy. It has been terrifying. My first baby was small and stayed in the low percentiles of growth until recently. She is totally normal and beautiful. My husband also has a really small head and can't wear adult size hats, but he's fine. I am worried but wondering if this doc just went down a worst case scenario spiral. Everything I've ready says these issues resolve in 75% of cases. The other cases are very bad but require the evidence of other issues. Has this happened to anyone else?

UPDATE: At a follow-up yesterday (02/17), the baby was determined to be still small but with larger measurements. The doc said that most of the issues resolve themselves, which would have been helpful information to include at the previous appointment. It was a great relief. Still additional follow-up appointments.