Actually Sherlock talks about Joan telling him that the adoption was about to go through. It was a major factor on why he broke it off as he told Gregson on the bench.
What bothers me is the actual number of times this great show had 13 episode seasons instead of 23. It got good numbers on some bad nights. No wonder the old networks are starting to fail. You can't keep shortening your shows or lowering the number of shows. I plant to start watching season 1 again soon, if I can find it.