A reader writes:
The reader who wrote that the IUD is a form of abortion is misinformed. But that's understandable; before my second year of medical school, I was too! When this topic came up in class, we had an extended conversation with our professor. Many of us were surprised and a little shocked to learn that pre-fertilization (spermicidal) effects were primarily responsible for the contraceptive activity of IUDs. Not totally believing our discussion, I followed up with a literature search. I was pretty convinced by what I read.
For example, if IUDs were primarily functioning to prevent implantation, you would expect small, transient increases in bhCG (the "pregnancy hormone") to be associated with IUD use, and you'd expect these transient increases to be more common in sexually active women using an IUD (indicating inhibition of implantation) than in women who are trying to get pregnant (indicating very early, even-before-you-know-you're-pregnant pregnancy loss). In fact, using a super-sensitive assay, researchers found that 1% of women on the IUD had transient increases in bhCG – compared to anywhere from 8 to 52% of couples trying to get pregnant. This means you're significantly more likely to lose a fertilized egg trying to get pregnant than using an IUD!
With routine use, the primary mechanism of action of an IUD is believed to be due to thickening of the cervical mucus (the spermicidal effect) and, in the case of Mirena, inhibition of ovulation. This is similar to the pill, which primarily prevents ovulation and thickens the cervical mucus, but which could also (theoretically) disrupt the endometrium to the extent that implantation is prevented. (To be fair, when an IUD is inserted shortly after coitus as a form of emergency contraception, its primary function is to inhibit implantation)
Bottom line: an IUD doesn't make the uterus inhospitable to implantation so much as it makes it inhospitable to *sperm.* However, for those for whom even the smallest risk of inhibiting implantation is non-negligible, take my professor's advice: stick to barrier methods of contraception (which could *really* do with a bit of evolution!).
"Cervical mucus" is a first for the Dish. Another writes:
Thank you so much for the running discussion of the IUD, an excellent form of birth control too frequently dismissed or overlooked in the United States. I have had a copper IUD (ParaGard) since 2003 and, despite some days of heavy cramps, have not regretted it for a second. It has consistently worked without the undesirable side effects of hormonal birth control that made me miserable for years.
I am writing to correct some misapprehensions about the IUD conveyed in your most recent post on this topic. Several of your correspondents claim that the IUD does not actually prevent conception, but only implantation of an embryo. This is an older belief about the IUD, but it does not reflect the most research on its contraception mechanism. While it is true that the IUD has effects on the uterine lining that would make implantation difficult, it also has deleterious effects on sperm that make them unlikely to ever get to fertilize the egg in the first place. This is particularly the case for the kind of copper IUD that I use. According to WebMD, "Copper is toxic to sperm. It makes the uterus and fallopian tubes produce fluid that kills sperm. This fluid contains white blood cells, copper ions, enzymes, and prostaglandins" (source).
See also Planned Parenthood's FAQ's: "Both the ParaGard and the Mirena IUDs affect the way sperm move, preventing them from joining with an egg. If sperm cannot join with an egg, pregnancy cannot happen. Both types also alter the lining of the uterus. Some people say that this keeps a fertilized egg from attaching to the lining of the uterus. But there is no proof that this actually happens."