Presence of prolactin hormone in the blood of lactating mothers suppresses the release of gonadotropins $(FSH$ and $LH$) from the pituitary gland. Gonadotropins are required for ovulation to occur. Thus, in the absence of gonadotropins, ovum is not released and hence chances of fertilisation are nil upto $6$ months following parturition (if the mother is regularly breast-feeding). Therefore, intensely lactating mothers do not generally conceive.