The unit requirements for the Labour calls are spot on.
Labour - Waters Broken. This mission (like the other 2) was introduced as part of the CP introducing CMs. This mission is to simulate an ambulance response, the CMs often would turn up before/after crews. Again, this is intended as an ambulance mission not CM only.
Labour - Premature Birth. All Labour calls where delivery is imminent require 2 x DCA, this is because there are 2 patients that may also need transporting separately. This means you can not change it to 1 x DCA and 1 x RRV. The chance for CC is because of the call itself, no explanation needed.
Labour - Home Birth. This mission was introduced to give the CMs something they could complete without the need for ambulances, hence why the chance first responders can take full care is set to 100. This would simulate a planned home Birth with no complications. 2x CMs is the right response as they would not work alone.
Child Birth. This mission predates the CMs. I will put this mission for removal.
RRVs - These only sit idle if you let them, depending on how you play the game.
RRVs are not just for critical care, any mission where the transport percent is below 100 has a chance to be completed by an RRV without the need for a DCA. Similarly these missions also have a chance to be dealt with any other first responder such as CFR/Co-Responder.
I will not put missions out that require RRVs instead of DCAs as this is unrealistic. The only reason the newest labour calls require 2 x DCAs is because of the nature of call and this falls in line with most ambulance trusts response to these calls.