Nursing mothers -- you can't be stopped from expressing milk at work, particularly if you are an hourly worker covered by the federal Fair Labor Standards Act.
The law says that mothers must be given time and space to pump their breasts to provide milk for their babies for up to a year. It doesn't necessarily have to be paid time, but if the employer already provides paid breaks and the mother chooses to use her break in that way, she must be compensated.
Also, mothers can express their milk as often as they need to -- anyone who has done this knows people's bodies can vary, by person, by the day and even by hour. The employer is obligated to provide a space (not the bathroom!) that is shielded from you and free from intrusions from co-workers and the public. Companies don't have to set up a permanent room for nursing, but they do need to have some room available whenever it's necessary.
There are some exceptions. Very small businesses not covered by the law's rules regarding breaks also do not have to provide the nursing break if they can prove that the provision imposes an undue hardship. The problem with that exception is that a mother who needs to express her milk is in no position to argue the point. If state laws provide more generous benefits, then the federal law can be pre-empted.