@Shibby they could depending on what you do, if food prep or Medical they can cite it as being against Health and Safety.
But on a whole they can do what ever they like, if they stipulate before you start that they have specific rules and you don't follow them and you are fired then you can't sue for damages due to the fact you were told :P
To be honest mothers always know when something is up. Its a sixth sense thats installed into them during their birth.
There will be deniability, there will be the usual questions of "are you sure?" "Is it a phase?". But one thing is for sure, the longer you keep quiet about it the harder it will be later on in life to tell them.