The dictionary-meaning of the word Ātreyī(Atreyi) is a ‘woman who has taken her menstrual bath’.. However, Nīlakaṇṭha (Nilakantha), the commentator of Mahābhārata (Mahabharata), takes this word as synonymous with ‘a pregnant woman’ . In Śāntiparva of Mahābhārata, it is mentioned that one who kills a woman who is Ātreyī, will commit a sin that is doubly greater than the sin of Brahmahatyā (Brahmahatya, killing a Brahmana) —
evantu samabhijñātāmātreyīṁ vā nipātayet
dviguṇā brahmahatyā vai ātreyīnidhane bhavet.
Here, since it is said that the sin will be twice greater than that of killing a Brāhmaṇa (Brahmana), it reinforces the meaning of Ātreyī to be taken as ‘a pregnant woman’. At that time, killing of a foetus or a child inside the womb was considered to be greater crime than murdering a Brāhmaṇa. In this regard, one who consciously kills a pregnant woman, actually destroys two lives, and probably that is why the crime is given such grave importance.