It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that in the days of ignorance, people used to sell the offspring of a pregnant camel in exchange for the offspring of another pregnant animal, and by the offspring of a pregnant animal is meant "the one who is still in the mother's womb," that is, its unborn child. The Prophet (peace and blessings be upon him) forbade this.