Why does the Mirena (levonorgestrel-releasing intrauterine system) cause spotting?

Why the Mirena IUD Causes Spotting

Spotting during Mirena (levonorgestrel-releasing intrauterine system) use is an expected side effect, particularly during the first 3-6 months after insertion, and is caused by the hormonal effect on the endometrium as it adjusts to the local levonorgestrel release. 1

Mechanism of Spotting with Mirena

• The Mirena IUD releases levonorgestrel locally into the uterine cavity, causing profound suppression of the endometrium, which leads to initial irregular bleeding patterns 2
• When first inserted, the IUD triggers a foreign body reaction in the surrounding endometrium, characterized by infiltration of inflammatory cells (polymorphonuclear leukocytes and macrophages) 2
• The levonorgestrel in Mirena causes thinning of the endometrial lining and development of dilated, thin-walled blood vessels, which can lead to irregular spotting 2
• Plasma concentrations of levonorgestrel are highest immediately after insertion (mean 260 pg/ml during first three months) and decrease over time (mean 129 pg/ml after one year), which correlates with the gradual improvement in spotting patterns 3

Timeline and Pattern of Bleeding

• Unscheduled spotting or light bleeding is expected during the first 3-6 months of LNG-IUD use and generally decreases with continued use 1
• Intermenstrual spotting is most common during the first 60 days after insertion 3
• Over time, bleeding generally decreases with LNG-IUD use, with many women experiencing only light menstrual bleeding or amenorrhea 1
• By 6 months of use, approximately 44% of women report amenorrhea, increasing to about 50% after 12 and 24 months 4
• Spotting is present in about 25% of users at 6 months, decreasing to 8-11% by 18-24 months 4

Clinical Management of Spotting

• Before insertion, patients should receive counseling about potential changes in bleeding patterns, emphasizing that initial spotting is expected and generally not harmful 1
• If spotting persists beyond 3-6 months and is concerning, clinicians should consider underlying gynecological problems such as:
  • LNG-IUD displacement
  • Sexually transmitted infections
  • Pregnancy
  • Pathologic uterine conditions (e.g., polyps or fibroids) 1
• If an underlying gynecological problem is found, treat the condition or refer for care 1
• Unlike with copper IUDs, NSAIDs have not been shown to be effective for treating spotting with LNG-IUDs 1
• No direct evidence supports specific therapeutic treatments for bleeding irregularities during LNG-IUD use 1

Important Considerations

• Enhanced counseling about expected bleeding patterns and reassurance that spotting is generally not harmful can significantly reduce method discontinuation 1
• Heavy or prolonged bleeding is uncommon with Mirena use and should prompt evaluation for other causes 1
• If spotting persists and the woman finds it unacceptable despite counseling, discuss alternative contraceptive methods 1
• Removals due to menstrual bleeding problems are most common in the first 6 months of use 4
• The continuation rate for Mirena is approximately 66.2% at the end of the second year, with good contraceptive efficacy 4

Differences Between Copper and Hormonal IUDs

• Copper IUDs typically cause more bleeding than levonorgestrel-releasing systems 5
• Copper IUD users have bleeding patterns that remain stable over time, while Mirena users often develop infrequent bleeding, spotting only, or amenorrhea 5
• Leucocyte migration is greater with copper IUDs than with inert IUDs, which may contribute to different bleeding patterns 2