Those who said: once she delivers one of them she is free to remarry

Al-Hasan said: When a man dies, or divorces his pregnant wife, and she gives birth to a child while another child is still in her womb, her waiting period ('iddah) is completed with the birth of the first child.

Ibrahim (may Allah have mercy on him) said: When she has given birth to one child, she becomes irrevocably separated (ba'inah).

Qatadah reports that Ikrimah said: When she has given birth to one child, she becomes irrevocably separated. He was asked: Can she marry now? He said: No.

Qatadah said: The servant (Ikrimah) was defeated in the argument.