The day after I don’t think she knew, but I got in trouble a week or two after for something else, and while going through my texts she saw the thread between me and the boy (ordering Uber’s, him making sure I was okay) and I’m sure she suspected it. I denied and denied and denied it, saying all we did was kiss. Within the next few days she took my to her OB/GYN and put me on birth control. After that she definitely knew I was having sex but she never asked about it. She would always ask me if I was pregnant and encourage me to take tests, and whenever I would schedule an appointment with my OB (whether it was an STD screening or just a regular checkup) she would always ask and make sure things were okay. My dad pressured her to put me on birth control and I think that when I was put on it, my mom knew that it would trigger me to continue the behavior. I am lucky my parents allow me to have privacy about my sex life. According to my other siblings, they say that as long as I am being safe and careful that my parents would prefer not to know. So, I don’t think that the next day your mom or dad is going to know immediately based on your actions or how you walk (unless you are acting guilty and walking weird still, which you shouldn’t) but I think that as you get older and mature and talk about boys (or girls) that they will catch on.
No, she doesn't know when it happened. But she does not I'm not a virgin. I had an apartment in college, and, of course, I had given a spare key to my parents. Well, me and my then boyfriend were spending a lazy afternoon engaging in sexual intercourse. My mom shows up on the scene, knocks, nobody answers, so she lets herself in. While she didn't get an actual view of the action, she certainly heard what was going on. I was mortified. Anyway, after the guy hurriedly left, she just smiled and said, "Don't worry, I won't tell your father; I was in college once, too." While I was relieved in one way, it sort of creeped me out to know my mother prim and proper mother was getting her share of penis from guys that weren't my father (they didn't meet until three years after she graduated college).
As far as my family and my public know, I'm still pure. Only my partners and I know otherwise.