137mm AC is 50th percentile for 19 weeks gestational age
158mm HC is 50th percentile for 18.5 weeks gestational age

- from these, I am assuming you were somewhere between 18.5 and 19.0
weeks at the time the scan was done (since you didn't mention them
saying these numbers were out of the expected range).

A BPD of 39mm at 18.5-19 weeks is going to be roughly 15th percentile.

This is not actually an abnormal measurement, but it is proportionally
smaller than the other measurements.

If the overall head size (HC) is normal but the BPD is small, that
does not suggest microcephaly or other anomalies associated with
grossly abnormal brain development. In these cases, the HC and BPD are
BOTH very small, usually less than 5th percentile.

Small BPD with normal HC suggests a normal head size, but a slightly
elongated head shape. This is very often found when the fetus is in
breech position - in which case it means nothing. Remember the fetal
head is flexible and can be compressed in certain positions and
there's nothing wrong with that.

There are some skull development abnormalities - synostoses and
dysostoses - that can result in very extremely elongated or otherwise
abnormal head shapes. At 19 weeks, it is really too early to be
picking these things up. Also, in these cases, the numbers are very
much more abnormal than the numbers you are quoting. I rather doubt
this is what is going on with your baby, but your providers are
scanning you serially just to make sure the baby's measurements
continue to grow normally.