Intensely lactating mothers do not generally conceive due to the

Intensely lactating mothers have high levels of prolactin in their body. High serum prolactin concentrations inhibit secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus, thereby decreasing the secretion of gonadotropins (luteinizing hormone and follicle-stimulating hormone), and may also inhibit the action of gonadotropins on the gonads.