SB 1.8 - Kṛṣṇa protects Uttarā
Kṛṣṇa wanted the Pāṇḍavas to rule the world because they were devotees. So He wanted to protect Parīkṣit who was within the womb of Uttarā. The Pāṇḍavas took up weapons seeing five flaming arrows heading towards them. Only the person at who a brahmāstra is shot can see it as it approaches, and so only the Pāṇḍavas saw it and no one else could. Aśvatthāmā had released five separate brahmāstras for each of the Pāṇḍavas. This was difficult to counteract because it would take time to release five brahmāstras. Kṛṣṇa, observing the great danger, at once took up His Sudarśana disc to protect His unalloyed devotees. The Lord had vowed not to take up any weapons during the battle of Kurukṣetra and even though the battle was over, still He should not have taken up any weapon. But the Lord is bhakta-vatsala, the lover of His devotees, and so He preferred to be that rather than a worldly moralist, who never breaks his vow.
The Lord of supreme mysticism resides within everyone’s heart. As such, just to protect the progeny of the Kuru dynasty, He covered the embryo of Uttarā by His personal energy (yoga-māyā). He protected the embryo and even Uttarā was unaware of what He did. Although the supreme brahmāstra weapon was irresistible, it was neutralized and foiled when confronted by the strength of Kṛṣṇa. Being the Absolute, Kṛṣṇa did not wait for any other’s help and foiled the weapon by His own energy. The child within the womb saw his protector, the Lord, as only thumb high and was all transcendental. In SB 3.3.17, Prabhupāda writes that the embryonic body of Parīkṣit was burnt by the brahmāstra of Aśvatthāmā but a second body was given by the Lord within the womb, and thus Parīkṣit was saved. The Pāṇḍavas also belonged to the Kuru dynasty, but when there were differences between the two families, the sons of Dhṛtarāṣṭra were known as Kurus whereas the sons of Pāṇḍu were known as Pāṇḍavas. Thus here Kuru means the Pāṇḍavas.
Nothing is impossible for the Lord but all His actions are wonderful for us, and thus He is always beyond the range of our conceivable limits. So it was not difficult for the Lord to counteract Aśvatthāmā’s weapon. After all by His own energy, the Lord maintains and annihilates all material things, although He Himself is unborn. By His power of omnipotence the Lord was present both inside and outside of Uttarā and other members of the Pāṇḍavas family.