Wouldn't it be just the 7 days or so sperm can live? I had mine out on May 18th BTW and the the month after is weird....I had like 2 periods and then what I thought was a third, but was actually very heavy bleeding for 17 days. Now I'm trying to chart and it's still messed up.
I've heard TTC after Mirena can take a few months but some one always gets pregnant the first month after. HTH.
Baby wearing Mamma to DS, dealing with all his allergies....and thriving w/another little bundle due in Early Feb