A marfu' hadith is reported from Abu Sa'id al-Khudri (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said concerning the captive women of Awtas: "A pregnant woman is not to be approached for intercourse until she delivers, nor one who is not pregnant until she has had one menstrual period." Al-Sha'bi narrated it from the Prophet (peace and blessings be upon him) as mursal.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15587