Congratulations on your pregnancy! Cancun airport has the same 'grey doorway' metal detectors as every other airport.
As for the manicure/pedicures, it depends where you get it done, if you get it done in one of the resorts or an 'up market' place near 5th Av. it will be expensive, around $40 (USD) but if you go to a smaller beauty salon further away from 5th Av. you can get manicures for around $15.