It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) conducted li'an (mutual cursing) even when the woman was pregnant.
Hadith Referenceمسند احمد / ومن مسند بني هاشم / 3339
Hadith Gradingحکم دارالسلام:حديث صحيح، وهذا إسناد ضعيف لضعف عباد بن منصور