I have 18 months between DS and DD and was pondering a third child (on exceptionally good days, even a fourth!).
When I saw one of the local GPs a while ago he told me his wife was expecting their third baby and said 'After our first we thought we knew what we doing and then had the second and realised what hard work it all is!'. I think he had 18 months between the first two and then a gap of 3 years between the second and third children.
So I was wondering what gaps mumsnetters have between their third (and fourth) children? Planned ones really. There's 9 and 11 years between me and my siblings (I'm the youngest) but I was 'an accident' so not really a good example.