The answer starts with "it depends."
How old are you? How hard was it to get pregnant the first time?
The younger you are, the easier it is to get pregnant and, in general, it may become harder to conceive as you age. In younger women, it isn't uncommon to see two pregnancies back to back, if no contraception is being used. And we have all heard about couples with years of infertility who finally has a baby with in vitro fertilization, and shortly after that pregnancy, they conceive spontaneously!
In general, there is no consistent answer to this question. We know that sometimes infertility may be related to stress, and the longer the period of infertility, the greater the stress. It becomes a vicious cycle. Then, when pregnancy does occur and the stress of infertility vanishes…Voila! The couple becomes very fertile and another baby is on the way shortly after the first baby!
The bottom line is that if you are young and you want to "space" your pregnancies, it is probably best to use some type of contraception between pregnancies. If you are older and want more babies, it is best not to wait longer than the nine months it takes for your body to replenish all the cells, minerals, and micro-nutrients it takes to have a healthy pregnancy.