IUDs primarily work by preventing fertilization.[40] The progestogen released from the hormonal IUDs may prevent ovulation from occurring but only partially.[41][42] The hormone also thickens the cervical mucus so that sperm cannot reach the fallopian tubes. The copper IUDs contain no hormones, but the copper ions in the cervical mucus are toxic to sperm. They also cause the uterus and fallopian tubes to produce a fluid that contains white blood cells, copper ions, enzymes, and prostaglandins, a combination that is also toxic to sperm.[41] The very high effectiveness of copper-releasing IUDs as emergency contraceptives implies they may also act by preventing implantation of the blastocyst.[43][44] In non-emergency use, prevention of implantation is at most an exceptional method of action, not a typical mechanism of action.[17]