Mechanism of Action
The mechanism of action of progestogen-only contraceptives depends on the progestogen activity and dose. High-dose progestogen-only contraceptives, such as injectable DMPA, inhibit follicular development and prevent ovulation as their primary mechanism of action. The progestogen decreases the pulse frequency of gonadotropin-releasing hormone (GnRH) release by the hypothalamus, which decreases the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) by the anterior pituitary. Decreased levels of FSH inhibit follicular development, preventing an increase in estradiol levels. Progestogen negative feedback and the lack of estrogen positive feedback on LH release prevent a LH surge. Inhibition of follicular development and the absence of a LH surge prevent ovulation.
A secondary mechanism of action of all progestogen-containing contraceptives is inhibition of sperm penetration by changes in the cervical mucus.
Inhibition of ovarian function during DMPA use causes the endometrium to become thin and atrophic. These changes in the endometrium could, theoretically, prevent implantation. However, because DMPA is highly effective in inhibiting ovulation and sperm penetration, the possibility of fertilization is negligible. No available data support prevention of implantation as a mechanism of action of DMPA.
Read more about this topic: Depo-Provera
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