Where the concern comes from
If an Rh-negative mother is carrying an Rh-positive baby, her body can develop antibodies against the baby's blood type. This is called Rh incompatibility, and it's a routine part of antenatal screening — not a reason to avoid pregnancy.
How it's managed today
Blood type is checked at the first antenatal visit as standard practice. When Rh incompatibility is a possibility, an Rh immunoglobulin injection (commonly known by the brand name RhoGAM) is used to prevent the mother's body from forming those antibodies in the first place.
This is well-established, widely used obstetric care — the point of writing this down is early, routine testing, not alarm.
If you're Rh-negative
The four Rh-negative types — O-, A-, B-, and AB- — each have a page with more on what makes that type distinct, including this note.
This is general information, not medical advice. Talk to an OB/GYN about what applies to your specific pregnancy.