L.M.
When I asked my doctor after my first was born, he said you are considered fertile as soon as your menses return. (I breastfed, so it took a long time for those to come back.) He did mention that there was a higher occurrance of low-birthweight babies if there's less than two years between gestations, but that's just statistics. There are many other factors that you and your doctor may want to weigh before deciding what is the best, safest course of action. I recommend talking with your doc first. He/she may refer you to a genetic counselor as well.
Best wishes to you.