Ask any third year medical student how hormonal contraception prevents pregnancy, and they’ll probably tell you it prevents ovulation. What they won’t tell you is that this effect is variable and dose-dependent, and if we depended on it alone, hormonal contraception would be much less effective.
That’s because of the very important, and in my opinion, much under-appreciated effect of hormonal contraception on cervical mucus.
A Cervical Mucus Primer
Fertile cervical mucus – which forms under the influence of rising estrogen levels in the first half of the menstrual cycle and is maximal around ovulation – is thin, watery, clear and easy for sperm to traverse.
Non-fertile mucus – which forms after ovulation and also in pregnancy under the influence of progesterone – is the exact opposite – thick,tacky, non-distensible and impossible for sperm to penetrate. (It’s not called the mucus plug for nothing…)
A Progestin Effect
Cervical mucus during hormonal contraceptive use mimics that of the second half of the menstrual cycle – scant, thick and impenetrable.
The responsible party here is progestin – molecules with names like levonorgestrel, medroxyprogesterone, norethindrone, norgestimate and desogestrel – which mimic the natural effect of progesterone on cervical mucus. Even in birth control pills that contain estrogen, this progestin effect dominates at the cervix.
This effect on cervical mucus explains why the progestin-only pill, Norplant and Implanon work so well to prevent pregnancy, even though ovulation can still occur during use of these methods.
Mirena IUD thickens cervical mucus
Now, a new study shows that the Mirena IUD also thickens cervical mucus and prevents sperm penetration.
This is not surprising, since Mirena contains the progestin levonorgestrel. But it is important information about how this method works to prevent pregnancy – in addition to impairing ovulation and fertilization, Mirena also prevents sperm from getting into the fallopian tube in the first place. One could argue that in fact, if sperm and egg never meet, the other contraceptive mechanisms of this IUD become unimportant, making the Mirena really mostly a local delivery system for progestin.
This effect on cervical mucus also may explain why users of the Mirena IUD have low rates of pelvic infection, a side effect that doomed the Dalkon Shield. (Of course, the Dalkon Shield also had a multifilament string that allowed bacteria to enter the uterus. Modern IUDs have monofilament strings.)
Could we thicken mucus without hormones?
I think a lot about this contraceptive effect of progestins, wondering if we could find a way to thicken cervical mucus locally without systemic hormonal exposure. The Mirena comes as close to a local effect as we have to date, although its progestin is still systemically absorbed and has body-wide effects.
But if we could develop a locally applied non-hormonal product that does the same thing, wouldn’t that be a great contraceptive option?
Great post and very informative, partically for the patient! Huge fan of the Mirena IUD. Just had the old one taken out (long overdue!) and a new one put in yesterday.
I usually recommend it to friends/family who have have kids already and don’t have any underlying health issues.
Agreed! Very interesting indeed. I’m not sure I ever really knew how the birth control worked.
The other day I walked by a woman on her cell and she said something about the cervix and thickening. I wonder if she was having a similar discussion (or maybe she’s one of your readers!)?
I hear a lot of good things about the Mirena IUD but, for some reason, I can’t bring myself to be comfortable with the idea of inserting things in me that stay there. Creepy.
Laura,
Hands down the best BC I have ever had. It didn’t bode well for one sister who had a history of Uterine fibroids and edometriosis. I was surprised her GYN said she would be a good candidate in the first place. Anyway, her IUD ended up expelling and coming out about 7 months after insertion and she had other issues as well. My other sister has had only positive experiences like myself.
As for me, the first time having it put in, it was a few days of uncomfortable cramping, then really erratic bleeding for about 3 months. Then things REALLY tapered off and I had no period for years. So nice. Oh the money I have saved on not having to purchase feminine hygiene products!
After having the first one taken out and the next one put in last week, I had bad cramping for one day and some bleeding, then nothing! Nice! In my case, I also found that I no longer had extreme PMS symptoms.
Anyway, it is not for everyone, and your Dr. should know that based on your history. I have only positive things to say about it!
I know that some GYN’s also use it as a method to control bleeding.
I knew this was a primary method of BC with Mirena but didn’t realize the same was true with combo BC pills. On my second Mirena, I’m well aware of the change in quality and quantity of cervical mucus. I’ve used POPs and didn’t experience the mucus changes. It’s fascinating! Thanks for the article.
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I habe a the mirena for almost three years i love it. I could say its the best i ever had and i cant complain i am not trying to have anymore kids and i only have two.
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