i have very low bp normally - typically around 85/60. my mom has been the same way her whole life also, so i imagine there is a genetic component to it.
it was a problem at two times during pregnancy #1 - at the very beginning of my second trimester, i passed out twice. what my ob told me is that around that time your blood vessels are expanding to accomodate the extra blood volume of pregnancy but that the blood volume itself has not increased yet. so most women's bp drops slightly around that time, and since mine was normally low, it was a problem. then i had an epidural during labor with #1, and my bp dropped very low, requiring oxygen and the fetal monitor that they attach to the baby's scalp since they kept losing her heartbeat with the external monitor.
pregnancy #2, same low bp, but i ate better and was sure to have a juice box and some trail mix with me at all times, and i didn't have an epidural (or an ob :LOL ).
long story short - keep food with you and keep the needle away