It is narrated from Aishah (may Allah be pleased with her) that a pregnant woman does not menstruate; when she sees blood, she should pray.
Ahmad bin Hanbal said: Yahya (al-Qattan) used to declare Ibn Abi Layla and Matar da'if (weak) in their narration from Ata.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15436