BH's are the practice runs your body takes before you go into labour.
Some can actually feel their tummy tightening, or may experience dull back pain. Bassically they are contractions but don't last.

When I think about it, I didn't have them with my first preg. But I may have just mistaken them for back ache??? But with my second I did get them, not as bad as some but I did have tightening for sure.