There is no universally accepted criterion for periodontal
disease at the patient level. Clinical signs such as gingival
inflammation (redness, bleeding, and edema), increased
probing pocket depth, and loss of connective tissue attachment
are its hallmarks, and in clinical practice are charted
for each tooth. It is not clear, however, how such site-bysite
measurements of periodontal disease should be combined
to yield a composite measure of the net periodontal
health of a patient. Before investigating the relationship of
periodontal disease with any outcome, including but not
limited to preterm birth, it is necessary to have a consistent,
rational definition of the disease that makes sense for
the population under study. In the present case, the definition
of periodontal disease was derived from data related
to the outcome of interest (i.e. PTB).