Dor Yesharim is a wonderful tool, but it is not infallible. A friend’s child and the prospective shidduch both checked with D”Y and were found to be genetically compatible – BUT – D”Y did NOT check for a gene that causes a particular non-life-threatening condition, and as it happened both the boy AND the girl carry this gene, and their children were born with the condition. All of genetics is in G-d’s hands, and while I strongly believe that D”Y has helped to eliminate a great deal of tzaar that might otherwise have occurred, nonetheless not all things are in our control even with D”Y.
My daughter and son-in-law checked around the second or third date, when they started to realize there was something potentially there.