Although I'm a single mum and waiting until things are working before having any more children, I personally want to wait until #1 is around 3yo before having #2. So I'd wait until DD is 2yo before starting to TTC. That's my opinion for my lifestyle. It depends on work and finances etc, so as long as its right for you or you can make the most of it, then it shouldn't matter. I just want DD walking and fully TT before the birth of #2. But it depends on what you make of it. Good luck to those TTC.