Quoting Jenna + 2!:" After your first child how/why/when did you decide to have more? What are the pros and cons of the age ... [snip!] ... If I wait till they are four like I originally planned I worry that it will be too big of a gap and they won't be close..."
I Like the 18 Month age gap, however I also think that a Mamma(& her Body mostly) really needs at least that Full Year between Pregnancies. so the best you would hope for would be 22months if you got pregnant immediately on your Child's first Birthday.
For us we "Set In Stone" that on their 1st Birthday we start Discussing another, & do the discussion(even if it's a foregone conclusion) for 3 Months before we "Allow" the "Let's Have Another" Decision to be accepted. End result is that our Daughter's EDD was Our Son's 3rd Birthday after nearly a Year of TTC.
Right now, we're Not necessarily trying to Prevent, but not (officially) actively TTC, but if we did manage to catch this Month 3.0 will be Due for our Daughter's 3rd Birthday.
I Suspect that if we did catch, Baby will be a Boy with an EDD the same as our Son, & also be 5 Days late(both my Babies were), on his 6th Birthday!