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?