This article is from back in June, but I just came across it.

http://www.theguardian.com/society/2014/jun/04/women-premature-birth-second-child-quickly

To summarize, basically the sooner a mother has a second child (risks are higher less than 18 months apart and even higher at only 12 or less months apart), the higher the risks of premature birth, birth defects, or childhood behavioral problems.

This was just interesting to me, because until women had wide access to birth control, "stair step" children were quite normal.

Would info like this influence your family planning at all?