Congratulations, Ruth7 for your coming marriage.
And I have to speak here because I am blessed with a wife who enjoys period sex. (In fact, I'm the "difficult" one because I don't like the "bloody mess" thereafter.)
She told me sex during her period relieve her a lot and make (painful) periods less painful. It also "regulates" her flow. (I don't really understand how she felt, but I know it does something good to her. Hence, I always comply to her wanting period sex.)
As recommended by previous poster, prepare a dark towel (big one) or something water proof to keep the sheets clean. In addition, have disposable towels nearby. Wipe before you move away from the sheets because (in my case) blood might drip and soiled the floor or uncovered sheets.
I don't know the scriptural view on period sex. If I remembered correctly, there is a Law (Moses) forbidding period sex. But I think that no longer apply to Christians. (Hope someone can clarify.)
Before I end, I like to say, from my own experience, period sex is different. Apart from the mess, I actually will give +1 (out of 10) more points than normal sex
I hope this experience of mine can encourage you NOT to be overly concern. Also, share this with your FH, assuring him (if he fears blood like me), it's going to be a "bloody experience" of a lifetime many don't get to enjoy
I love my wife, that's why I crave to have sex with her.