If a woman has an undiagnosed sexually transmitted infection (STI) at the time an IUD is placed in the uterus, she is at a higher risk of getting pelvic inflammatory disease (PID). This is why health care providers often require that you take an STI test before getting one. This is especially likely if you are under age 26, have a new sexual partner, or have multiple partners—because all these groups are at higher risk of having an STI.
About 20 days after having an IUD inserted, a woman is past the danger zone for developing PID from an STI she didn’t know she had during the insertion. After that time, PID happens at the same rate for women using IUDs as for those using other methods of birth control (aside from condoms).