Levonorgestrel - how does it work?

[BlondeCookie]

levonorgestrel - a progestin which is slowly released in a continuous flow into the uterus. This causes thickening of the cervical mucus, thins the uterine lining and prevents a fertilized egg from implanting in the uterus.

I thought that Progesterone converts the endometrium to its secretory stage to prepare the uterus for implantation. So, why does the levonorgestrel (a progestin) prevents the egg from implantation?
😕

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[BBCatcherVA]

My simplified explanation in my head was that progestins stabilized endometrium that was thickened by estrogens and that chronic levonorgestrel exposure inhibited LH/FSH secretion, thus inhibiting estrogen secretion, thus preventing endometrial thickening, thus preventing egg implantation. However, since you asked the question I decided to try to look it up. You might want to check out this article at http://humupd.oxfordjournals.org/cgi/reprint/10/4/341. I know it focuses on emergency contraception mechanisms of action, but it seemed like a good review. Hope this helps.🙂

 

[aldolase]

I am really confused about the oral contraceptives and their MOA. What happens if unprotected sex takes place after ovulation when progesterone is making the uterus ready for implantation ? wouldn't giving progestin only pill like plan B enhance it's action ?

 

[THP]

BBCatcher is essentially correct. Estrogen prepares the endometrium for implantation whereas progesterone stabilizes it and prevents it from sloughing off, thus maintaining a pregnancy. If there is no pregnancy then the progesterone falls and you get a period. You can see this empirically by placing a post menopausal woman on estrogen only and over time she will develop a thickened endometrial lining. If you place a woman with a thickened endometrial lining on daily progestin for an extended period of time her lining will become very thin. The primary MOA for OCPs is their progestin effect more so than the estradiol. Most women on OCPs, especially continuous OCPs will have a very thin lining.

Plan B works because the large dose of progestin prevents ovulation. It is ineffective at preventing pregnancy after ovulation. I suppose it could theoretically cause some changes to the endometrium that would prevent inplantation but I not aware of evidence for that.

Mirena prevents implantation in the same way Paragard dose - it's related to inflammation and foreign body effect. This is not primarily how Mirena works however.