It's said the IUD (Mirena) is the most effective birth control device. But it costs $380 to buy the device. It's hurt when the doctor put the device in. There will be bleeding/spotting up to the first 3 months. Irregular spotting may appear later on.
Your period duration will be reduced. With time going by, your period may totally disappeared unless you remove the device.
It's said the device is not harmful for your later childbirth. You can be pregnant 1-2 years after you remove the device. I heard a friend got pregnant 6 months after the device removal (she inserted the ring-shaped device in China.) And her kid is very healthy.
But I will not choose to insert the device unless I seriously decide not to be pregnant anymore.