Well, I had sex with my partner after the 18th day of my mensuration cycle. We didn't use protection. And I have no idea that if he left any sperm in there. I want to know the consequences and what do I do if my next period time will overdue. And is there any medical pill to stop pregnancy after 4-5 weeks?

The consequences should be obvious, the consequence is pregnancy. ANY time there is penile insertion there is the risk of pregnancy. No form of birth control is 100%.

There are pills that will abort an early pregnancy, but they have to be prescribed by a physician.

If your next period is late, you wait about 2 weeks and then take a pregnancy test.

It is possible that you could be pregnant. If he was inside, he left sperm. It does not all ever come out. Even if just inside for a little bit, there can be sperm in the early fluids.

There are pills to use within the first 2 or 3 days, but not at this point.

Now you have to confirm that you are pregnant and then talk to your doctor about ways to end it.

Having sex and no idea of consequences? It called motherhood! Too late for next day pill. See an OB. And get on the pill!