Why is Maria al-Qibtiyya considered a slave?

Please give a response to this as I can not find an answer anywhere. Was Maria al-Qibtiyya a slave before giving birth to prophet's son? Prior to Islam there were many sources of slavery but after Islam the only source of slavery is captives captured in war or children of slaves. Maria was neither of them, she was gifted by a king and she belonged to a noble family in that place. How did she become slave in the first place when she was not obtained in war. Just because a king claimed she was a slaves, will she became a slave for Muslim? Isn't the only source of slavery being captured in war or children of slave?