I had boys 19months apart ( as on, I got PG 9 months after normal delivery.). Physio sucked her teeth and said that my various ligament problems were to do with mybody not having had time to 'bounce back'
My wife just had a VBAC (2nd child) with a gap of 17 months between our two boys.
People talk about scar rupture but it is a really tiny risk. Having spoken to the obstetric consultant about it, the risk of uterine rupture is 5 in 1,000 (1/200) in a normal VBAC. That goes up to 8 in 1000 if the labour is accelerated using Syntocin. However, if labour is induced it goes up to 25/1,000 (1 in 40), which seems pretty high to me. On the other hand, for a labour after a section, you can always choose another section or convert a natural labour to section at any time at your election. Even those odds above overstate things as there are ruptures with and without sequelae and the statistics are not separated. And, finally, according to our midwife, she has seen two ruptures in 18 years of midwifery and they all came after long stop/start labours. As she put it (in what seems logical to me) if you are not opening up where you should (the cervix), you will open up somewhere else (the scar).
I don't think people should plan their family structure based on the risk of uterine rupture. On the other hand, a third child within 18 months?! I don't think I could cope with that personally.