The vesatot are designed to limit the time frame during which a woman must be cautious about becoming niddah. When your vesatot have passed on a given cycle, there is no longer a halachic restriction on having relations, even though you still expect your period.

In these cases, you should use your best judgment. If you suspect your period is imminent you can check before intercourse for any bleeding by wiping externally with a tissue (not immediately after urinating).