Treatment with NORPLANT^R subdermal implants inhibits sperm penetration through cervical mucus in vitro

Attempts were made to collect several samples of cervical mucus in each of thirty-three cycles of women using NORPLANT^R subdermal implants, in seven cycles from NORPLANT-2 rod users, and in 33 control cycles. The attempts to collect a mucus sample were successful on 20 of 77 and 7 of 14 occasions in NORPLANT^R capsule and rod users, respectively, due to the scanty amount and stickiness found in the majority. All 59 attempts in control subjects were successful. The distance travelled by the fastest sperm and by the bulk of spermatozoa through the cervical mucus in vitro was assessed after 10 min of incubation with a normal semen sample. The best score for each cycle was considered for the comparison between treated and control cases. Sperm penetration was greatly impaired in mucus samples of implant users with penetration by the fastest sperm exceeding 10 mm in only two instances and exceeding 20 mm in none. In 30 of 33 control samples, the fastest sperm travelled 21 mm or more and in 28, the bulk of spermatozoa travelled further than 10 mm. Unsuccessful attempts to collect mucus and poor sperm migration were observed in NORPLANT^R implant users even when circulating estradiol levels were comparable to those seen in the late follicular phase of the normal menstrual cycle. These results suggest that the principal mechanism by which NORPLANT^R implants prevent pregnancy is by interference of sperm migration through cervical mucus.