Yahya related to me from Malik from Nafi that Abdullah ibn Umardid not sacrifice for the child in the womb.Malik said, "Thesacrifice is sunna, and it is not obligatory. I prefer that anyone whohas the price of the animal should not abandon it.''
USC-MSA web (English) reference: Book 23, Hadith 13
Arabic reference: Book 23, Hadith 1044