How does chronic periodontitis affect pregnancy?

Several things: Among other things, it could result in preterm labor or low birth weight. The body is stressed enough during pregnancy. Bacterial infection adds to that. Also, the increase in Progesterone levels during pregnancy cause soft tissues to relax; this includes your gums. Even otherwise healthy gums may swell and bleed during pregnancy. Periodontitis could flare up and get worse during this time.
Can be dangerous. An elevated level of strep mutans can be carried by the blood and pass the placental barrier.
Premature labor. Interestingly, inflammation or infection of the oral cavity can actually increase the risk for premature labor. The theory is that bacteria from the infected tissues in the mouth can get into the bloodstream and be carried to pregnancy, and infect the bag of waters or the placenta. Good dental hygiene is crucial to a good pregnancy outcome!