Data collection and analysis

We prospectively collected data on all fetal RHD tests performed in the first 15 months of the national screening programme (4 July 2011 to 7 October 2012) and cord blood tests until 31 December 2012 directly from our laboratory database. Statistical analyses were performed using GraphPad Prism version 5.01 for Windows and software from www.medcalc.org. For fractions close to boundary values of 0 or 1 we calculated 95% confidence intervals using a correction by Fleiss.²³ The performance of the fetal RHD test was monitored every four weeks (independent of sample size) to enable adaptations to the antenatal screening programme if the false negativity rate would exceed the preset limit of 0.25%. Compliance to fetal RHD testing was also calculated every four weeks for one year after implementation, using the data from the central database Praeventis. We calculated compliance for women with an expected date of delivery between 30 September 2011 and 28 September 2012, corresponding to the 27 weeks’ gestation mark occurring between 1 July 2011 and 29 June 2012. For samples from 112 women without a registered fetal RHD test result and with an expected date of delivery in January or February 2012, we contacted the obstetric care provider to ask for information on the decision not to perform fetal RHD testing.

Data were available for the following descriptive variables: maternal age, ethnicity, parity, gestational age at time of sampling, and cell-free fetal DNA level (table 1​1).). The variables maternal age and cell-free fetal DNA level were available in the fetal RHD test dataset. The fetal RHD test dataset was linked to clinical data of the perinatal registry in the Netherlands for the purposes of a different study. This was done only for pregnancies with a cord blood serology result. We extracted data on ethnicity, parity, and gestational age at time of blood sampling from this linked and anonymised dataset.

Overview of population characteristics

*Not including Mediterranean countries.

†All countries surrounding the Mediterranean Sea.

‡Black Creole includes all black people.

§Asian included all Asian countries, with the exception of people of Hindustani descent.