In one scene Joker goes on about Bats being distracted and careless because of the baby,so to fix it he has to kill the baby. Robin asks Batgirl what he is talking about and Batgirl blows him off. In certain canon (namely the Bruce Timm and Paul Dini) Batman and Batgirl are known to have had an affair. Could joker be referring to Batgirl possibly being pregnant? Food for thought