Darrow began to research HDN with a dedication that her colleagues described as consuming her;[1]: 29-31  she later stated that after her son's death, "the study of this disease has been my chief avocation".

The immunization may conceivably occur as a result of an accident whereby the fetal cells or their hemoglobin gain entrance to the maternal blood sinuses.

In 1938, while working as a physician at the Women's and Children's Hospital in Chicago,[5] Darrow published a review of HDN in the Archives of Pathology that drew on her personal experiences in addition to the existing literature.

The maternal antibody could then cross into the fetal circulation and cause an antigen-antibody reaction resulting in destruction of red blood cells.

[4] While her description of the general mechanism of HDN was correct, Darrow believed the causative agent was fetal hemoglobin rather than blood group antibodies.

[8] She based this hypothesis on studies by Louis K. Diamond which showed that the parents of babies with HDN usually had compatible ABO blood types; the Rh antigen was poorly understood at the time.