Speaking as an ex-dental assistant.....
Extra hormones and increased blood supply are to blame for the propensity to have gums bleed during pregnancy. But don't stop brushing/flossing just because of the bleeding (which may or may not occur ~ it's not true that every woman will have this happen). The chemical changes in the body affect the bacteria in the mouth, making a pregnant woman more susceptible to decay (and thus providing ammunition for the belief that having babies will "rob" calcium from your teeth ~ which is patently false. Calcium, once deposited in your teeth prenatally, cannot be removed. It can be leached from your bones, but not your teeth).
Rinsing gently with water after brushing can ease any soreness in the gums, and make sure you brush thoroughly, but gently, to not irritate the tissue excessively. The situation will resolve after the baby's born.
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