The time it takes for an IUD to take effect depends on the type of IUD and when during your cycle it was placed. ParaGard prevents pregnancy as soon as it is inserted. If Mirena, Kyleena, Liletta, or Skyla are placed within the first five days after the start of your period, they are also effective immediately. However, if your IUD was placed after that, you should use another birth control method for the first week.