A few things about IUDs — those flexible, T-shaped birth control devices — are certain. For one thing, they’re about 99 percent effective at preventing pregnancy.

They’re also supposed to make your periods lighter. Some people will find that their monthly flow becomes a thing of the past.

But everyone’s experience — and subsequent bleeding — is totally different. There are so many possible variables that it’s impossible to predict exactly how your body will respond.

Here’s what you should know.

Will the IUD spare you from having monthly periods? Your odds of having to continue buying pads or tampons may depend on how heavy your pre-IUD periods were.

Researchers in one 2016 study looked at more than 1,800 people who used the Mirena IUD. After a year, those who’d started out with light or short periods were more likely to stop bleeding altogether.

While 21 percent of participants with light periods reported that their menstrual flow stopped, only 5 percent of those with heavy periods had the same results.

There are four hormonal IUDs — Mirena, Kyleena, Liletta, and Skyla — and one copper IUD — ParaGard.

Hormonal IUDs may make your periods lighter. Some people don’t get periods at all while on them.

Copper IUDs often make periods heavier and crampier. However, this may not be a permanent change. Your period may return to its usual state after about six months.

Hormonal birth control can throw off your menstrual cycle. At first, your periods may be heavier than usual. Eventually, the bleeding should get lighter.

What to expect from insertion to 6 months

For the first three to six months after your IUD is placed, expect the unexpected when it comes to your periods. They may not come as regularly as they once did. You could have some spotting in between periods or heavier-than-usual periods.

The length of your periods may also increase temporarily. About 20 percent of people bleed for more than eight days in their first few months after insertion.

What to expect from 6 months on

Your periods should get lighter after the first six months, and you may have fewer of them. Some may find that their periods continue to be more unpredictable than they were in the past.

About 1 in 5 people will no longer have a monthly period by the one-year mark.

Copper IUDs don’t contain hormones, so you won’t see changes in the timing of your periods. But you can expect more bleeding than before — at least for a while.

What to expect from insertion to 6 months

In the first two to three months on Paragard, your periods will be heavier than they were before. They’ll also last longer than they once did, and you may have more cramps.

What to expect from 6 months on

The heavy bleeding should let up after about three months, putting you back into your normal cycle routine. If you’re still bleeding heavily at six months, see the doctor who placed your IUD.

You may typically avoid going to a gynecologist while you’re on your period, but IUD insertion is different. Your doctor may actually want you to come in while you’re bleeding.

Why? It’s partially about your comfort. Although an IUD can be inserted at any point in your cycle, your cervix may be softer and more open while you’re on your period. That makes insertion easier for your doctor and more comfortable for you.

Being on your period also assures your doctor that you aren’t pregnant. You can’t get an IUD if you’ve already conceived.

Having an IUD during pregnancy can cause serious risks to both you and the fetus, including:

Getting a hormonal IUD inserted during your period ensures that you’ll be protected right away. Hormonal IUDs are immediately effective when inserted during menstruation.

During the rest of your cycle, it’ll take about seven days after insertion for a hormonal IUD to start working. You’ll need to use additional protection — like condoms — during this time to prevent pregnancy.

Because the copper itself prevents pregnancy, this IUD will start to protect you as soon as your doctor inserts it. It doesn’t matter where you are in your cycle.

You can even insert a copper IUD up to five days after unprotected sex to prevent pregnancy.

See the doctor who inserted your IUD if you experience:

  • unusually heavy bleeding beyond the first six months
  • fever
  • chills
  • abdominal pain
  • pain during sex
  • foul-smelling discharge
  • sores on your vagina
  • severe headaches
  • yellow skin or in the whites of your eyes (jaundice)

Your periods should settle into a normal rhythm after one year. A small percentage of people using a hormonal IUD will stop getting a period altogether.

If you haven’t gotten a period for six weeks or more, call your doctor to make sure you’re not pregnant. They’ll assess your overall symptoms and administer a pregnancy test to confirm that you’re not pregnant.

If the test is negative, you shouldn’t need to return unless you begin experiencing early pregnancy or other unusual symptoms.

Once your IUD has been placed, you don’t have to do anything. Just check your threads once a month to make sure the IUD is still in the right place. Your doctor can show you how to do this.

If you can’t feel the threads, call your doctor. Although it’s likely the result of the strings curling upward, the IUD itself may have shifted position. Your doctor can confirm correct placement and answer any other questions you have.

Otherwise, see a doctor for annual checkups to confirm placement.