Hi!
The BPD is at the lower limits of normal for that gestational age--there could be very many factors influencing the measurement though--the MOST likely being the technique of the sonographer. I almost NEVER get concerned about measurements obtained on only one scan, the first thing I would do is wait 3-4 weeks and repeat the scan. It is very likely that the baby is just fine. Also, the placenta moves up the uterine wall as the baby grows and the uterus expands, so in close to 99% of cases it is well out of the way by the time the baby is due. As far as carrying the baby low, there are lots of wive's tales about the meaning of this, but medically there is no significance.
Be sure to ask your own doctor about all of this too!
Good luck!
Dr B