There is 20mths between my first three and I think it was the best gap to have and worked wonderfully for us for the same reasons Tanya and Julie have said. But in saying that, there is just over 3yrs between #3 and #4, and that also works great too as I had that break from babies kwim? Paige has been fully TT since she was 2.5yo and is a good helper most of the time LOL. She doesn't have a regular day sleep, but if I really needed her to sleep, she would, so from my perspective I have had a really good run with having a close gap and also a bigger gap.