They are currently available only for women, although hormonal contraceptives for men have been and are being clinically tested.[35] There are two types of oral birth control pills, the combined oral contraceptive pills (which contain both estrogen and a progestin) and the progestogen-only pills (sometimes called minipills).[36] If either is taken during pregnancy, they do not increase the risk of miscarriage nor cause birth defects.[3] Both types of birth control pills prevent fertilization mainly by inhibiting ovulation and thickening cervical mucus.[37][38] They may also change the lining of the uterus and thus decrease implantation.[38] Their effectiveness depends on the user's adherence to taking the pills.[33]