Imam al-Shafi'i (may Allah have mercy on him) said: "And it was clear from the revelation of the verse that it is not lawful for a divorced woman to conceal the menstruation that is in her womb, and this also includes the possibility of pregnancy along with menstruation."
And it is narrated from Sa'id bin Salim, from Ibn Jurayj, from Mujahid (may Allah have mercy on him) that he said, concerning the saying of Allah, the Mighty and Majestic: "And it is not lawful for them to conceal what Allah has created in their wombs" [Surah al-Baqarah: 228]: "It is not lawful for a divorced woman to say: I am pregnant, when she is not pregnant; nor: I am not pregnant, when she is pregnant; nor: I am menstruating, when she is not menstruating; nor: I am not menstruating, when she is menstruating."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / Q15413