Thanks for the query.
belongs to category B FDA.
Diphenhydramine is only recommended for use during pregnancy
only when benefit outweighs risk.
So i recommend you not take tablet only if it is needed. Dont make it a habit to take every night.
Some studies have shown side effects like cleft palate
when received in first trimester.
It can also increase uterine contractions when received in last trimester.
Hope this helps,