As with most personal information, it's not always appropriate to share something intimate with a person you haven't known for very long. Boys aren't always super mature at this time in their lives, and if it makes him shy, confused, or uncomfortable he might not know how to react.  If you have been dating for a while and you feel you know him pretty well, you might want to bring it up casually. You don't need to make a big deal about it, because it really isn't a big deal! Think about how he's reacted to other potentially awkward moments in the past. Did he freak out, embarrass you, or tell all his friends? If so, you might want to hold off. Is it having an impact on your relationship? Do your cramps keep you from going out with him? Do you get wicked mood swings and snap at him? If so, it might be a good idea to clue him in so he knows it's nothing personal. If it's not really affecting your relationship, then it's entirely up to you whether or not you want to tell him. If you want to keep it private, that's totally fine. If you want to be up front about it, then go for it. Chances are, he's not going to have any idea what you are talking about and the misunderstanding might make things unnecessarily awkward. If you choose to tell him, use clear language but make sure he knows it's something personal you are choosing to tell him. Say something like "I know I've been moody this week. I've actually got my period right now and sometimes it makes me a little sensitive. I just wanted to let you know what was going on with me--I'd really appreciate it if you didn't mention it to anyone else." Periods can be pretty mysterious and intimidating to boys, and he might just blush and say "Oh. Um. Okay." and never bring it up again. It's not cool, however, if he makes fun of you or says it's gross, but you shouldn't take that personally--it's his problem he's acting like an immature little kid. There's absolutely nothing wrong with you or gross about your period--in fact, it means you're perfectly healthy.  You can...

How would you rephrase that briefly using Vietnamese?
Cân nhắc xem hai bạn hẹn hò được bao lâu. Suy nghĩ xem tại sao bạn muốn nói cho anh ấy biết. Tránh dùng cách nói gián tiếp để ám chỉ chu kỳ kinh nguyệt của bạn như "tuần dâu rụng," "ngày đèn đỏ," "tới tháng," hoặc bất kỳ cách nói tránh nào. Đừng cảm thấy bị tổn thương nếu anh ấy trốn tránh cuộc trò chuyện.