I went away and thought about this for a bit, here's what I think. Feel free to tell me where I am wrong!
Okay, say that there are 100 things of equal probability that could happen in a year. One being I get pregnant, the other 99 being I don't. 1/100.
For two years, there are 10000 things of equal probability that could have happened... 100 for each of the 100 things that happened in the first year. 100 of these ten thousand are possible babies, and a further 99 are the chance that I had a baby last year and none this year. 1 is the chance that I have had a baby each year. That makes 200/10000... or 2/100. 2%.