This is something I read a lot. If you are in that situation, know the following:
Both of your parents must have either been Rh(D) negative or carried that trait recessive.
The D antigen in the Rh blood group system determines whether or not you are rh negative. If you are rh negative, you will be market as dd. Rh D positive homozygotes as DD and heterozygotes as Dd.
As the d is recessive, it can be passed down silently for generations until partnering with another’s d to create a dd.