After Trump's election, getting an IUD flew to the top of many people's to-do lists. According to a recent analysis, demand went up nearly 20% in November and December of 2016. And because Paragard, aka the copper IUD, can last for over a decade and doesn't come with the sometimes troublesome side effects of hormones, it's a very tempting option. Throw in the fact that a copper IUD can also be used as a form of emergency contraception, and bam — it makes sense for a whole lotta uteruses. But the way it actually works is much less obvious than its hormonal siblings.
Before we get too deep, let's take it back to Sex Ed real quick: When sperm fertilizes an egg, the resulting blastocyst implants in the walls of your uterus. That little blastocyst eventually grows into a fetus. One way hormonal IUDs, such as the Mirena, work is by preventing the hormone surge that triggers ovulation — the release of the egg into the fallopian tubes. As Planned Parenthood puts it, "No egg, no pregnancy." Hormonal IUDs also cause your cervical mucus to thicken, making it harder for sperm to reach an egg were it somehow released anyway.
Copper IUDs, on the other hand, don't really mess with your hormones directly. Instead, the copper wire that wraps around them releases copper ions into the surrounding fluids of your uterus. The IUD also causes your body to increase the amount of white blood cells and prostaglandins (hormone-like compounds) in these uterine fluids. When the powers of those white blood cells, prostaglandins, and copper ions combine, the mucus cocktail acts as a spermicide. It's literally toxic to sperm, and keeps them from getting to the egg, which prevents fertilization.
When you're using a copper IUD for emergency contraception, however, the picture is lot less clear. If inserted within five days of having unprotected sex, a copper IUD can prevent pregnancy (it's actually the most effective form of emergency contraception around). We know this works in 99.9% of cases, but considering that sperm can meet an egg within five days of unprotected sex, it's possible that something besides fertilization prevention is going on here.
In those rare cases when the sperm has already met its egg by the time a copper IUD is inserted, some researchers think a copper IUD may instead prevent implantation by making the uterine lining less hospitable to a fertilized egg, Mario Ascoli, PhD, tells Vox. Some conservative groups consider this to be on par with abortion, and, therefore, classify copper IUDs as abortifacients. But, again, these are rare cases. And no mainstream medical group considers copper IUDs to be anything other than contraceptives — even when used in emergency situations.
Of course, no one type of contraception will work for everyone. But knowing how they work can help you make a decision that feels right — for the next four years and beyond.