They will not be effective as birth control. If you’re pregnant, you’re not ovulating monthly, so there’s nothing for them to do.
They won’t affect menstrual timing, flow, and symptoms because your hormone levels will be far above what the pills contain. In fact, those high natural hormone levels put you at far greater risk of blood clots—especially after delivery—than any oral contraceptive pill ever made.
So far, there has been no case of a human fetus or baby injured by a mother’s use of oral contraceptives while pregnant, and given the much lower hormone amounts in modern pills compared to those of a generation ago, there’s no reason to suspect this to change.