Racism and medical neglect of both women and black people play a major part in the poor outcomes for black women's pregnancies. However, it's a lot more complex than "just" getting worse care in pregnancy.
By the time black women in the UK get pregnant, they are often affected by a lifetime of socioeconomic disadvantage (on average - not all black women, obviously), which in turn affects their health. So, on average, they are in poorer health than other groups - again, this is not to deny racism: on the contrary, I'm saying that racism affects black women's health throughout their life. This is even more true in the US because of the cost of healthcare.
So, of course we need to try to address health inequalities and racism during pregnancy but we also need to do this for the whole of people's lives.