Bay City, Michigan 1929

Looking for names of parents and any siblings of baby girl born 5/20/29 in Bay City Michigan. Adoptive parents named her Mary Louise. This may have been her birth name. There is a rumor that her adoptive father's younger sister was her birthmother. No other adoptive siblings. Adoptive parents, aunts and uncles are all deceased. Not sure how to access original birth certificate of baby born in 1929. This is pre Baby Scoop Era.