Study design and population
This study was part of the OPZI 2.0 study, a nationwide cohort study on RhD immunization in pregnancy. All pregnant women with a positive screening for RhD antibodies at any moment in pregnancy, identified at Sanquin Diagnostic Services during our study period, were eligible for inclusion. Positive screenings as a result of a RhIg administration were not included. Women were identified from two time periods (for practical reasons): from July 1, 2014 to March 31, 2015 and from August 1, 2015 to February 28, 2017. Women were excluded if the mother additionally had another antibody with a titer higher than that of RhD (and an antigen-positive child).
The local care provider of eligible pregnant women was contacted in order to obtain patient’s informed consent. Subsequently, clinical data were collected from the care provider in a detailed questionnaire. If outcome data were incomplete, the researchers made at least three attempts to contact care providers or study participants directly to complete the questionnaire. Missing data on receiving RhIg in a previous pregnancy was obtained from the Department for Vaccine Supply and Prevention Programs (RIVM-DVP).
To test the hypothesis that HDFN is more severe in the subsequent pregnancy with RhD immunization than in the first immunized pregnancy, we selected all women with at least two pregnancies with RhD antibodies and RhD-positive foetuses from the OPZI 2.0 cohort. In order to assess the risk of selection and non-response bias, characteristics of included and non-included cases were compared (supplemental text).