1. Fluoride Large studies demonstrate safe use in pregnancy.
Inhibits bacteria growth and strengthens enamel.
Used topically to prevent dental caries.
Dietary fluoride supplements for pregnant women is not recommended as there is not added benefit for the fetus; topical fluoride rinses are what is needed to derive the above benefits.
Xylitol No direct studies in pregnancy have been performed, but studies where it has been used as an intervention have not demonstrated harm.
Used in gum, xylitol:
Decreases level of mutans streptococci in saliva and plaque by interfering with cellular metabolism
Selects for less virulent strains of mutans streptococci
In addition, chewing sugarless gum stimulates saliva which assists with neutralization of acids and re-mineralization of tooth enamel.  
Chlorhexidine Vaginal application studies have shown no harm to the fetus. No studies of oral use are available, but exposure would be similar when used as a mouthwash. Absorption from the gastrointestinal tract is poor.
Used as mouthwash to decrease periodontal and cariogenic bacteria
Reduces gingivitis and plaque deposition
References
Ly KA, Milgrom P, Rothen M. Xylitol, sweeteners, and dental caries. Pediatric Dentristry 2006;28:
American Dental Association Council on Access, Prevention and Interprofessional Relations. Women’s Oral Health Issues Available at:
Motherisk. Toronto, Ontario, Canada at (which included referencing National Collaborative Perinatal Project, , Record Group 443, National Institutes of Health.)
Briggs GC, Freeman RK, Yaffe SJ. Drugs in Pregnancy and Lactation, Ninth Edition. Lippincott Williams and Wilkins, Philadelphia, PA 2011.