If there were 2 populations of cells, and if the fetal cells were ABO incompatible with the maternal sample however D positive, the fetal cells could have been hemolyzed in vitro by the time the sample was retested 10 hours later. Since the fetal cells were no longer intact, the repeat testing would have been on exclusively maternal cells and tested as D-negative. I realize your initial forward type did not detect/report the presence of mixed field agglutination. With monoclonal ABO antibodies and using tube testing, it is often very difficult to detect mixed field reactions with small populations of other cells. It seems the monoclonal ABO antibodies are so avid, the agglutinates seem to "trap" the other population of cells in the agglutinate rather than remaining free to be resuspended. This was demonstrated years ago when the AABB offered "damp" workshops where samples were provided for testing, registrants reported their results and results were tallied and presented to attendees. During the time when the older polyclonal ABO reagents were still in use and the monoclonal reagents were just being introduced, the results indicated a significant difference in the detection of mixed field agglutination in the forward ABO testing between the groups using polyclonal reagents (MF detected) vs. monoclonal reagents (MF not detected).