Life of sperm is only 72 hours, so you can get
pregnant up to 72 hours after intercourse and the test that is the BCG levels in which pregnancy is tested comes after 14 to 21 days of the pregnancy. So a maximum time is around 14 to 21 days of intercourse. If your
pregnancy test is negative after 21 days of your last intercourse, that means you are not pregnant and irregularity of periods is due to hormone imbalance.