To ensure there are seven complete days, we begin the first day at sunset and end the seventh day at tzeit. This means the seven clean days end on the same day of the week they started. During this week, a woman must make sure she remains free of blood. To do this, she checks herself internally every day with a bedika cloth. She also wears white underwear for the week. If she finds even a drop of blood on the cloth, or if she finds a stain of at least two centimeters diameter in her underwear, she must do a new hefsek tahara before sunset and start counting the seven clean days again.
If a woman knows that her period has ended, but she sometimes bleeds anyway (whether because of an IUD, hormonal pills, or vaginal cuts), she should check herself only twice during the seven clean days. She should also wear colored underwear, to avoid seeing stains that might cause halakhic difficulties.
