My girlfriend take birth control regularly. but, she takes zoloft for anxiety. can this reduce the effectiveness of her pills and cause a pregnancy?

In theory : Risk of unintended pregnancy associated with combining antidepressants and birth control pills is more theoretical than realistic.Antidepressants such as SSRIs can affect levels of estrogen and progesterone in women thus affecting the bioavailability, or amount of circulating hormone in the body, of these hormones change, the effectiveness of oral contraceptives.In practice,this is rarely a case.