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).