My ex and I did this for pretty much the whole time we were together and I fell pregnant pretty much every 2 years.

First was in 2001.
Second in 2003.
Briley in 2005.

Colby was concieved in 2006 but I was trying to fall pregnant with her. Given the space in between all the other pregnancies I had thought it would take some time to fall pregnant again. This wasnt the case and she was conceived when Briley was just shy of 4 months.

I was again 'playing with fire' since having the Mirena taken out in 2009 and fell pregnant in April last year.

So I am guessing that for me, when there isnt a contraceptive in play I fall pregnant every 2 years when doing the whole 'trying but not trying' thingo.