A birth control diaphragm or cap is a reusable barrier method of birth control that is inserted into the vagina before sex. It is covered in spermicide (a gel that kills sperm) and prevents pregnancy by covering the cervix so sperm can’t get into the womb. There are a few things that can make the diaphragm or cap less effective. These include if it doesn't fit properly,
you remove it too early,
it is damaged,
you forget to use spermicide,
or it is used with other oil-based products. If these things happen you may need to use emergency birth control.