36 Weeks and 2 days - NST Appointment

So, I'm on my second pregnancy. I'm having a boy this time and I have a two vessel umbilical cord. I've been considered high risk so every 3 weeks I have a growth ultrasound and an NST twice a week since he is growing a little on the small side, but then again my fiancé and I are both in between 5'0 and 5'5. Anyway, I was told everything's growing good and healthy, but at today's NST appointment, my little bun's heart rate dipped after a contraction then went back up and was fine the rest of the time.