What way to keep a healthy placental function?

Honestly, this is something worth discussing properly with your gynae or O&G specialist rather than relying on general advice, because placental health and preeclampsia risk really depend on your individual situation. Some doctors do talk about a balanced diet with enough protein, fruits, and vegetables as a general foundation, but the specifics vary from person to person. I would not want to guess at what works best for you, since getting it wrong during pregnancy is not worth the risk. Bringing this list of questions to your next antenatal appointment is probably the safest and most useful thing you can do.