Can you take birth control pills after eighteen days of your last period?

yes you can. Birth Control in any for is going to redirect your cycle to 28 days no matter what. but if you want you period to come 3 weeks from your regular period( when you are on birth control you will not have a period, it is called withdrawal bleeding. because you don't ovulate while on birth control. withdrawal from the hormone causes the bleeding.) so actually you can choose when you"period" comes. hope this helps!!!