Dry ‘Intrauterine Swimming Pool’ for the Sperm” – A Potential New Mechanism of Action of Levonorgestrel-Releasing Intrauterine System (LNG-IUS, Mirena) as a Contraceptive

Findings: In the literature, the mechanisms of action of LNG-IUS as a contraceptive include impairment of sperm transport through the cervix due to thick cervical mucus and through the uterus due to inflammatory changes in the endometrium affecting sperm transport and function, and possibly fertilisation and implantation are impaired. The evidence from the literature shows that following long-term use of LNG-IUS the changes in the endometrium is predominantly atrophy and decidualisation not inflammation, the latter being a feature during the initial phase after its insertion. As endometrial atrophy is the predominant feature of long-term LNG-IUS use, impairment of the sperm transport through the uterus due to inadequate volume of endometrial fluid might play a major role in the mechanism of action for its contraceptive effect.