Congenital syphilis, why isn't it always transmitted from infected Mother, or is it?

Learning about the lyme epidemic and it's relationship to the syphilis spirochete, I find it hard to believe that it isn't always transmitted from the Mother to the fetus. It is often difficult if not impossible to detect in a newborn, and can take years to diagnose. Syphilis is blood born, which is enough to reliably transmit it. Furthermore the spirochetes can drill through virtually all tissues including into bone.

Are most/all Mothers routinely screened for syphilis? How is it that an infected Mother can form a complete fetus without transmitting this highly infectious disease? Tweet