There was another powerful weapon called Brahmashira. Brahmashira was never used in war. It was four times more powerful than Brahmastra. This was because Brahma had four heads, thus the name. Only Arjun and Ashwatthama possessed the knowledge to summon the Brahmashira.
Arjun had the power to retract this, but Ashwatthama did not. After Ashwatthama killed Uppandavas, a fight ensued between him and Arjun. During the fight, Ashwatthama invoked the extremely powerful Brahmashira against Arjun. Arjun in response invoked the same astra. Fearing the destruction of the world, the sages advised both to take back their weapons. While Arjun could do it, Ashwatthama could not.
Therefore, he was given the option of choosing any single target to destroy. Out of spite, Ashwatthama directed the weapon to the wombs of Pandava women. Among them was Uttara, Arjun’s daughter-in-law. Lord Krishna saved the unborn Parikshit, but cursed Ashwatthama for his cowardly act.