I was in the same boat, however after reading this book and charting for a few months, I know exactly what is going on.
First I wondered "am I ovulating?" I got my answer after one cycle, yes I am. Great.
Then, I saw that it was taking my body a long time to ovulate and that I was ovulating much later in the cycle than before leaving me with too short a luteal phase (time between ovulation and period) for implantation.
So, I cut back from feeding 2yo ds on demand to 4 or 5 feeds a day, and bingo, ovulating much earlier and having a long enough luteal phase now. Hopefully will be pg very soon
So, the bottom line is this. You may not have to stop to get pregnant but on the other hand, you might. If you follow the methods in this book then you will have an answer as to whether you need to stop rather than stopping because you think you need to IYSWIM.
But maybe you just want to stop anyway, which is fine too, 16 months is brilliant!