Is your current job so unbearable that you got to resign straight away? After the 16 weeks of ML, it takes some time to assimilate back into the working life. Coupled with taking care of your new baby, it can be very hectic and takes time to cope with all the drastic changes. Morever, if you were to change to a new job, you would need to start the learning curve all over again, and this may means more stress and OTs in the initial stage. Not only that, your new boss may not be supportive of you taking pumping breaks as well as leave for baby's injections,checkups and such.
I would think it's better to "ren" for at least till baby is around 6~8 months old before making a switch.