When a divorced pregnant woman gives birth, the divorce becomes irrevocable

It was narrated from Zubair bin 'Awwam that:
he was married to Umm Kulthum bint 'Uqbah, and she said to him when she was pregnant. "I will accept one divorce." So he divorced her once. Then he went out for prayer, and when he came back she had given birth. He said: "What is wrong with her? She misled me may Allah mislead her!" Then he came to the Prophet ﷺ, who said: "Her waiting period is over (and she is divorced); propose marriage a new to her."

حَدَّثَنَا مُحَمَّدُ بْنُ عُمَرَ بْنِ هَيَّاجٍ، حَدَّثَنَا قَبِيصَةُ بْنُ عُقْبَةَ، حَدَّثَنَا سُفْيَانُ، عَنْ عَمْرِو بْنِ مَيْمُونٍ، عَنْ أَبِيهِ، عَنِ الزُّبَيْرِ بْنِ الْعَوَّامِ، أَنَّهُ كَانَتْ عِنْدَهُ أُمُّ كُلْثُومٍ بِنْتُ عُقْبَةَ فَقَالَتْ لَهُ وَهِيَ حَامِلٌ طَيِّبْ نَفْسِي بِتَطْلِيقَةٍ . فَطَلَّقَهَا تَطْلِيقَةً ثُمَّ خَرَجَ إِلَى الصَّلاَةِ فَرَجَعَ وَقَدْ وَضَعَتْ . فَقَالَ مَالَهَا خَدَعَتْنِي خَدَعَهَا اللَّهُ ثُمَّ أَتَى النَّبِيَّ ﷺ فَقَالَ " سَبَقَ الْكِتَابُ أَجَلَهُ اخْطُبْهَا إِلَى نَفْسِهَا " .