"With the copper IUD, periods may be somewhat heavier and crampier than usual," Dr. Schaffir says. Because copper IUDs don't involve hormones, they don't stop you from having a period, and people who've had one fitted often report a heavier period, particularly in the first few months. It's not known exactly why this is — it could have to do with blood flow in your reproductive system, or irritation to your uterus — but stock up on heavy-duty pads or be prepared to empty your menstrual cup a lot.

"With the hormonal IUD, periods are lighter, sometimes to the point where a period does not come at all," Dr. Schaffir says. This is relatively common; in 20% of people who have the Mirena hormonal IUD, according to the Food and Drug Association (FDA), periods disappear entirely. This is why hormonal IUDs are sometimes recommended to help people deal with heavy periods.

"Rarely, IUDs may be related to some discomfort during intercourse," Dr. Schaffir says. In a small study published in Obstetrics & Gynecology in 2016, 17% of people with IUDs reported sexual dysfunction in the first three months after their IUD was inserted, citing increased bleeding in particular. (It's worth noting that 40% of people reported positive changes in their sex lives, too!) Your sexual partner might also feel uncomfortable if they can feel the strings of the IUD, Dr. Gersh adds, but it's basically impossible for sex, no matter how rough, to dislodge an IUD.

If a person with an IUD gets an STI, the infection is more likely to affect their reproductive system and result in a painful pelvic infection, or Pelvic Inflammatory Disease (PID), Dr. Schaffir says. Studies show that this remains pretty rare — and an IUD won't cause PID on its own — but it's a good idea to use condoms and dental dams with new partners, even with birth control.

Hormonal IUDs, Dr. Schaffir says, are more likely than copper IUDs to cause breakthrough bleeding. "This is light bleeding that occurs unrelated to a period," he says. It's not just hormonal IUDs, though. A study published in Open Access Journal of Contraception in 2016 found that up to 70% of people with copper IUDs experienced breakthrough spotting in the first three to six months.

Sometimes, Dr. Gersh says, the IUD doesn't stay in the uterus, and begins to fall out. This happens in around 10% of people who get an IUD within the first three years, according to a study published in Obstetrics & Gynecology in 2014. Scientists don't agree on why it happens, but your OB-GYN might have placed the IUD badly, your uterus might be an odd shape, or there could be other factors involved. One study published in Contraception in 2017 found that younger IUD users are more likely to experience expulsion than older ones. If you think your IUD is falling out — maybe you can feel it poking into the bottom of your cervix, or the strings are much lower than they used to be — schedule a trip to the OB-GYN immediately.

In unusual cases, Dr. Gersh says, the IUD can decide it wants to embed itself in your uterus. This is really rare; a study published in Open Access Journal of Contraception in 2016 found that it only happens in about one in 1,000 IUD insertions. You'll know that something's off if your IUD is perforating your uterus, because the muscle walls will start cramping and you'll experience pain. It will likely have to be removed surgically.

This might be a really, really uncommon side effect of hormonal IUDs, Dr. Gersh says. It's a condition where the pressure of the fluid surrounding the brain gets too high, causing serious headaches and, in some people, loss of sight. "This is rare," Dr. Gersh says. One 2015 study published in Therapeutic Advances In Drug Safety found that a hormonal IUD appeared to be linked to a higher risk of intercranial hypertension. But a lot more studies need to happen before scientists understand how or why.