All pregnant women have a blood test to determine whether they are rhesus positive or negative.

http://www.patient.co.uk/health/ectopic-pregnancy-leaflet

 All pregnant women have a blood test to determine whether they are rhesus positive or negative. The injection of anti-D immunoglobulin simply prevents you from producing antibodies, which can be harmful in future pregnancies, if you are rhesus negative. You do not need this injection though if you receive medical treatment.