Here is what I know about child spacing:
Hunter-Gatherers had their children 3-6 years apart and usually had 4-5 children.
What this information means to me is that my body evolved to function best having babies every 3-6 years and to have 4-5 children, but that doesn't mean, should I want to have twelve babies in twelve years, that my body could not do it.
However, in the 1920's Weston A. Price spent over a decade traveling the globe looking for the healthiest people in the world (a control group to which he could compare Americans). He recorded the dietary and lifestyle habits of the thirteen or so native groups he considered to be the healthiest people in the world and noted that they followed this ancient pattern of spacing children no closer than three years. He learned that the native people believed that a baby born closer than three years after a sibling was considered to be unhealthy. He looked into this and found evidence enough to convince him of the validity of this concern. In the photographs he includes in his book Nutrition and Physical Degeneration, one can see the perfectly straight, white teeth of siblings spaced 3 or more years apart and the crooked teeth of the sibling born too soon. Crooked teeth were just one signifier of in-womb nutrition deficiencies though--narrow faces, narrow hips leading to more difficult births, club feet and almost all other birth defects were more common in children born closer than three years.
Price's theory was that pregnancy exhausts a woman's nutrient stores. Nursing a baby further depletes those stores (or slows down the replenishment process). It takes a woman's body several years to replenish and be ready to give everything to a new baby. A baby born only a year or two after a sibling will most likely not be able to receive enough nutrients in the womb to develop properly.
There are many other things that can deplete a woman's biological fitness and make it not wise to have a baby. Children born soon after a woman suffers from a major illness or during a time of famine also showed signs of not getting properly nourished in the womb. This did not mean these babies couldn't survive, it just meant their gene expression was not optimized.
To put this theory into a real life example: very few Americans today, even the healthiest, will have children with naturally straight teeth. But if any of their children have straight teeth, it the most likely be the couple's first born. This could also be why the first born will have the highest IQ, be the most attractive and the least likely to have a hormonal imbalance. (If a woman's nutrient stores are properly replenished before she gets pregnant again, it is possible for all her children to be attractive with straight teeth and high IQs. Again, this does not mean that babies whose gene expression was not optimized will be stupid and unattractive, it just means that had they been properly nourished in the womb, they would have been even more intelligent and attractive than they are.) I cannot find any examples from people in my life in which this is not the case. Whenever I run into people who look like they have nice, wide mouths and perfectly straight teeth I ask about their mothers' diets before they were born. 100% of the time thus far, the person with the straight-teeth-no-braces had a mom who ate a traditional diet or some strange traditional foods that most people don't eat today or, in one case, fished and ate fish for at least one meal a day every day.
For these reasons, I would never consider having children spaced closer than three years.
Why did we lose our ancient knowledge and start having more children spaced closer together? The change took place after the middle ages due to religions pressure to not nurse and have large families. Nursing is a natural contraceptive. Native societies nursed their young for 3-6 years. When the church convinced women that nursing was sinful and dirty and babies started being weened either at birth or after a month, infant mortality skyrocketed (thus women had to have more children in order to have one or two survive to adulthood) and women were able to get pregnant again right away. The ancient knowledge, that this would lead to unhealthy offspring, was lost... and crooked teeth, narrow faces, and difficult child-bearing hips became normal.
If you would like to read more about this subject, check out:
Nutrition and Physical Degeneration
Centuries of Childhood: A Social History of Family Life