Darryll - we only ever discussed having two children. Lo and behold I conceived very easily, had easy pregnancies and produced a matching pair - DS 6 and DD 3.
The trouble is, that after DS (i.e. even before DD), I started yearning for three; visualising three; enviously gazing at women with three children etc. Whenever I thought of the kids, I started to imagine three.
I mentioned this to DH and he said "you must be joking!!!". So I didn't mention it for another two years, but my broodiness got worse. As soon as I gave birth to DD, I wanted to get PG again.
About 18 months ago we had a "talk" and I told him that if he didn't let me have another baby, I would regret it for the rest of my life, probably resenting him as well. I explained that I'd been thinking about it for over two years and the yearning wouldn't go away.
He was pursuaded and it took us a few months for me to conceive. Then I miscarried and we had to start again, and it took me even longer to conceive. But DH is very supportive now and has been since he realised how desperately I wanted a third child.
I think its best to be honest rather than get PG accidently on purpose, as that may make DH/DP mistrust you or feel he's been tricked.
In practical terms it is not ideal for us having a third, but like I said, I think I would always regret it if we hadn't tried, and would always feel there was a little person missing.
BTW the little person is due in March
Good luck with it Darryll