SB 10.2.21

kim adya tasmin karaṇīyam āśu me
 yad artha-tantro na vihanti vikramam
striyāḥ svasur gurumatyā vadho ’yaṁ
 yaśaḥ śriyaṁ hanty anukālam āyuḥ
Synonyms: 
kim — what; adya — now, immediately; tasmin — in this situation; karaṇīyam — is to be done; āśu — without delay; me — my duty; yat — because; artha-tantraḥ — the Supreme Personality of Godhead, who is always determined to protect the sādhus and kill the asādhus; na — does not; vihanti — give up; vikramam — His prowess; striyāḥ — of a woman; svasuḥ — of my sister; guru-matyāḥ — especially when she is pregnant; vadhaḥ ayam — the killing; yaśaḥ — fame; śriyam — opulence; hanti — will vanquish; anukālam — forever; āyuḥ — and the duration of life.
Translation: 
Kaṁsa thought: What is my duty now? The Supreme Lord, who knows His purpose will not give up His prowess. Devakī is a woman, she is my sister, and moreover she is now pregnant. If I kill her, my reputation, opulence and duration of life will certainly be vanquished.
Purport: 

According to Vedic principles, a woman, a brāhmaṇa, an old man, a child and a cow should never be killed. It appears that Kaṁsa, although a great enemy of the Supreme Personality of Godhead, was aware of the Vedic culture and conscious of the fact that the soul transmigrates from one body to another and that one suffers in the next life according to the karmas of this life. Therefore he was afraid of killing Devakī, since she was a woman, she was his sister, and she was pregnant. A kṣatriya becomes famous by performing heroic acts. But what would be heroic about killing a woman who, while confined in his custody, was under his shelter? Therefore, he did not want to act drastically by killing Devakī. Kaṁsa’s enemy was within Devakī’s womb, but killing an enemy in such a nescient state would not be an exhibition of prowess. According to kṣatriya rules, an enemy should be fought face to face and with proper weapons. Then if the enemy is killed, the victor becomes famous. Kaṁsa very conscientiously deliberated upon these facts and therefore refrained from killing Devakī, although he was completely confident that his enemy had already appeared within her womb.