Without getting too graphic, after a wedding night, the kallah is likely to have some bleeding. Does this affect the nidda status of the kallah? If so, is it immediate? How could that be? Does it begin the next day?
YD §193 is about this. It is too comprehensive a discussion for me to adequately address here. Some basic points, however:
Blood which comes from a wound - דם מכה - does not render a woman a Niddah. Technically speaking, hymeneal bleeding - דם בתולים - is blood from a wound. However, due to certain Halachic concerns, various amora'im (and some tanna'im before them, to a lesser degree) decided to look at hymeneal bleeding as typical menstrual fluid that renders a woman a Niddah. Later on another chumra was imposed that a virgin becomes a Niddah after her first intercourse even if there is no hymeneal bleeding, out of concern that there was blood and it just wasn't noticed. As a rule, however, these stringencies are not put into effect until after the first intercourse is completed, even if there is blood.
The exact Halachos and how they apply nowadays is a complicated subject, and it is incumbent on anyone entering that phase of life (mazal tov!) to find a rabbi who can elaborate on all this in detail, and in a practical manner.
As an aside, reading this teshuva is crucial for anyone learning the sugya.