I have the Kyleena IUD and use that method alone. People get pregnant while on birth control for various reasons. Most of the time it is due to personal error and people not knowing how the pill works. For example, many people aren’t aware that throwing up/having excessive diarrhea can have any impact on the effectiveness. There is also a rare genetic mutation that breaks down the hormones in birth control which would make it ineffective, but you would know very early on if that were the case!