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Questions persist about the mechanisms by which the levonorgestrel-releasing intrauterine system (LNG-IUS) prevents conception. Progestin-only contraceptives hamper sperm penetration by altering cervical mucus, but prevention of sperm penetration in LNG-IUS users has not been documented. In a nonrandomized investigator-blinded study, qualities and in vitro sperm-penetrability of cervical mucus from 14 LNG-IUS users were compared with those from 16 controls who used no hormonal contraception. Commercially banked sperm was obtained from one donor; cervical mucus was collected at midcycle as demonstrated by presence of a luteinizing hormone surge within 24 hours before collection.
As evaluated with scoring criteria developed by the WHO, cervical mucus from LNG-IUS users, compared with that from controls, was substantially less likely to favor sperm penetration. A simplified glass slide test showed sperm penetration of cervical mucus from 0% of LNG-IUS users versus 64% of controls (P<0.001). A different in vitro test (involving penetration density and migration distance) showed that, within 2 hours of incubation, sperm penetrated mucus from 0% of LNG-IUS users versus 85% of controls.
Lewis RA et al. Effects of the levonorgestrel-releasing intrauterine system on cervical mucus quality and sperm penetrability. Contraception 2010 Dec; 82:491.
Comment
Attenuated permeability as shown by in vitro assays using sperm from one donor hardly constitutes hard evidence that the same holds true in vivo; nonetheless, these data provide additional support that the LNG-IUS does not act primarily by preventing implantation. Together with older results indicating that other intrauterine devices elicit alterations in sperm motility, these data show that the LNG-IUS probably prevents fertilization by a variety of mechanisms; such findings should prove helpful to clinicians when explaining to patients how the LNG-IUS works.