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Any foreign object in the uterus could be a place where bacteria could attach and cause an infection - Most people would recommend you removing it after expiry... nothing may happen, but I wouldn't want my wife to have an expired Mirena in for a long time because I wouldn't want her to develop some kind of uterine infection.
That being said, severe 'cramping' is unlikely due to your Mirena.... even though it 'expires' after 5 years, the hormones can last a lot longer than that - its just that it is GAURANTEED to last 5 years.... what is most likely happening is that the Mirena hormones have finally completely 'ran out' and the cramping is most likely from that.
I know you already paid for your doctor's visit; however, if you paid 900 bucks for a replacement mirena, it makes sense to get it placed and the other removed. while not emergent, I would suggest doing this in the next few months once you have money saved up.
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