It is narrated from Alqamah bin Qays that Abdullah bin Mas'ud (may Allah be pleased with him) said: "Whoever wishes, I am ready to invoke a mutual curse (mubahalah) with him that the verse 'And for those who are pregnant, their term is until they deliver their burden' [Surah al-Talaq: 4] was not revealed except after the verse in which the husband's death is mentioned; so when the woman whose husband has died gives birth, her waiting period has ended." By the verse of 'the woman whose husband has died' he meant this verse: 'And those of you who die and leave behind wives, they shall wait concerning themselves for four months and ten days.' [Surah al-Baqarah: 234]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15475