Our blood type can be positive or negative. The rhesus factor refers to this. If a pregnant lady is rhesus negative there are certain investigations, explanations and management necessary.
Read about the biology to understand the risk with rhesus negative. Understand the medical approach with testing antibodies during the pregnancy and administering rhogam during or after pregnancy.
Here are some interesting resources and explanations. You can also read more in the relevant sections in the text books in our library.