Do you know where she is?

You can call the Police and report her missing. They may or may not locate her, depending on how concerned you are, if she left a note.

You can also go to Family Court in your area, explain the situation and ask for their advice. They cannot give you legal advice but they CAN make suggestions - such as file for custody, visitation, something else.

They will attempt to locate her OR at some point she has to come back for clothes and supplies for the baby.

You are living together, she is the mother, you cannot stop her from living apart from you at this time.