A mother cannot conceive when she is intensely breast feeding because

Lactational amenorrhea (absence of menstruation) method is based on the fact that ovulation and therefore the menstrual cycle cannot occur during the period of intense lactation following parturition. As long as the mother breast-feeds the child fully, chances of conception are almost nil. It is effective up to six months.

High levels of circulating prolactin inhibits the secretion of GnRH and thereby gonadotropins.