Могу ли я пропустить месячные после секса 17 числа, если они должны были начаться 20-21 числа? Вопрос!

Stress is usually the biggest factor

I used to have such bad irregular periods that I’d miss my period for like 46 days.

If your having sex and not using condoms or a form of birth control, there is always a chance of pregnancy. Take a pregnancy test if it's negative wake another week then test if that's negative and still no period call your Dr and get a check up if that is not normal for you. Our body's can miss periods for lots of reason or be late.

Well did you have sex at all before the 17th? Because it’s unlikely to get pregnant 4 days before your period is due. Your fertile window already passed. It would’ve had to happen earlier.