S.K.
It is one of the assessments a doctor or midwife makes of the pelvis during the physical exam (along with things like ischial spines, side walls, sacral promontory, etc). Usually during the initial prenatal exam the provider takes measurements of the pelvis, (not formal measurements like with a tape measure, but rather with their hand/fingers) to get an idea of weather or not the pelvis is favorable for a vaginal delivery. A "classic" female pelvis has a subpubic angle of about 90 degrees, a narrower arch COULD make for a more difficult vaginal delivery. It is just one of many factors a provider may take into consideration at the end of the pregnancy or during labor. It alone would not be a reason for an early induction or scheduled c-section, just something they might keep in mind if they feel you have a large baby or labor is not progressing well. It is possible that you will need to consider "alternative" positions to facilitate labor, but it is certainly not something to stress about right now. It probably was not mentioned because is is such a small factor by itself, it is not anything you can change, there is no decision to be made, and it simply may case undue worry or stress for you. Good luck with the rest of your pregnancy.