It is narrated from Ali (may Allah be pleased with him) concerning a man who divorced his wife while there were two children in her womb; she gave birth to one and the other remained. He said: He has more right to take her back as long as she has not delivered the second one as well.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15438