What is the primary mechanism of action of levonorgestrel in the Mirena intrauterine device?

• A. Increase estrogen uptake and decrease sperm motility
• B. Thicken the endometrium and cervical mucus
• C. Suppress gonadotropic hormones
• D. Decrease the movement of ovum through fallopian tubes

Answer: (B)

The primary mechanism of action of levonorgestrel in the Mirena intrauterine device is to thicken the endometrium and cervical mucus. This action plays a crucial role in preventing pregnancy. The thickened cervical mucus makes it more difficult for sperm to enter the uterus and reach an egg, thereby reducing the likelihood of fertilization. Additionally, the changes to the endometrium create an environment that is less favorable for implantation should fertilization occur.

While there are other potential mechanisms at play, such as suppressing ovulation or affecting endometrial receptivity, the thickening of cervical mucus is a significant and rapid local effect that directly influences the effectiveness of this intrauterine device. This is why it is considered the primary mechanism in the context of levonorgestrel's action within the Mirena IUD.