The Islamic Shari`ah has called for honoring man and every part of him whether alive or dead. `Aishah, may Allah be pleased with her, reported that the Prophet (peace and blessings be upon him) ordered his followers to bury seven parts of the human being; hair, nails, blood, menstruation blood, teeth, foreskin and the placenta. This is to honor man and all his body parts, and to avoid any pollution that is caused as a result either to man or to the environment around him. Muslim scholars state that body parts such as the placenta are to be buried in case there is no apparent benefit for leaving it unburied. If, on the other hand, there is an apparent benefit that can be accrued as a result of having the placenta used in a beneficial way that is doesn’t contradict with the basic rulings of the Shari`ah, then there is nothing wrong with that.”
Moreover, Sheikh Muhammad ibn Salih al-`Uthaymeen, the prominent Saudi scholar, states the following when asked about using the placenta for treating diseases like cancer and the like: “Such parts are to be buried in the same way as we do with nails and hair. However, there is nothing wrong with making use of it in treating diseases so long as there is proof that it is an effective treatment.”
Based on what is mentioned above, we can conclude that there is nothing wrong in donating the placenta to the hospital to conduct research on it that is beneficial to treating diseases. Thus, what you did from donating the placenta is a praiseworthy act and you will be rewarded for it, in sha’ Allah.