Smallest IUD Available
The 13.5 mg levonorgestrel IUD (Skyla) is the smallest intrauterine device currently available in the United States, approved for 3 years of use. 1
Available IUD Options by Size
The United States currently has five IUDs available, with varying sizes and hormone content 2:
Levonorgestrel-Releasing IUDs (Smallest to Largest)
- 13.5 mg levonorgestrel IUD (Skyla): The smallest option, releasing 14 mcg per 24 hours, approved for 3 years 1, 3
- 19.5 mg levonorgestrel IUD: Mid-sized option 2
- 52 mg levonorgestrel IUDs (Mirena, Liletta): Larger devices releasing 20 mcg per 24 hours, approved for 5-7 years 1, 3
Copper IUD
- Copper T 380A: Non-hormonal option, approved for 10 years but effective for at least 12 years 4
Why Size Matters: The Anatomical Reality
Most nulliparous women have uterine cavities that are narrower than conventional IUDs, making smaller devices particularly important for this population. 5
Key Anatomical Findings
- The mean uterine cavity width in nulliparous women is only 22.2 mm (range 6.0-41.1 mm) 5
- 32% of nulliparous women have cavity widths less than 20 mm 5
- 6.8% have cavity widths less than 15 mm 5
- Mean endometrial cavity length is 3.84 cm in nulliparous women versus 4.25 cm in parous women 6
Clinical Implications of Size Mismatch
When IUDs are too large for the uterine cavity, women experience higher rates of 7, 5:
- Expulsion
- Embedment
- Bleeding and pain
- Early discontinuation (5-year continuation rates only 40-50%)
Clinical Recommendations
The smaller 13.5 mg levonorgestrel IUD (Skyla) is particularly appropriate for nulliparous women and adolescents who may have smaller uterine cavities. 1, 2
Safety in Nulliparous Women
- The American Academy of Pediatrics confirms that levonorgestrel IUDs are safe for nulliparous adolescents 1
- IUDs do not cause tubal infertility in nulliparous women 1
- Rapid return to fertility occurs after removal 1
Consideration for Preprocedural Imaging
Preprocedural 2D or 3D ultrasound to measure uterine cavity width may help select the most appropriate IUD size and maximize continuation rates. 5 This is especially relevant given that one-third of nulliparous women have cavity widths less than 20 mm, which may be too narrow for standard-sized devices 5.
Common Pitfall to Avoid
Do not assume all women can accommodate standard-sized IUDs. Nulliparous women have significantly smaller uterine cavities than parous women, and inserting oversized devices leads to preventable complications and discontinuation 6, 5.