It depends how small, your birth weight and ethnicity, I.e: Asian women have smaller babies. Your baby should be about your birth weight. They should get you a scan to see if the baby is small and if it is how the placenta is doing. My baby was small due to my placenta (iugr but don't Google that you'll get scared and mine stayed in for weeks and is perfectly fine now), they monitored and got her out early, but the placenta doesn't suddenly fail, so they have to check, but you need a doppler scan I would think.
When they measured me last visit I was measuring 2 weeks smaller. My doctor wasn't concerned either. I'm just hoping for a smaller baby to pass lol. I'm almost 40 weeks.