If you do not use birth control, you will end up with children who are too closely spaced. Even if you breastfeed each child exclusively for the full 6 months (which is what is necessary, barring psychiatric medications, HIV in mother, or other serious circumstances), and, not conceive within those 6 months (exclusive breastfeeding for the first 6 months is a very effective birth control method), you will still end up with children who are too closely spaced.

Spacing the children too closely has a whole host of negative consequences, both physical and psychological, for a number of people involved - the child who was conceived too soon after the immediately preceding child was born (physical and psychological consequences), the mother (physical and psychological consequences), and all the older siblings (psychological consequences). Possibly, there are even negative consequences for the father if he gets overwhelmed with having children so frequently, and if his own needs end up being perpetually on the back burner.