It is permissible for a man to marry a second wife and as many as four wives, because Allaah says (interpretation of the meaning):
“then marry (other) women of your choice, two or three, or four; but if you fear that you shall not be able to deal justly (with them), then only one”
This is subject to the condition that he be financially and physically able for it, and able to treat them fairly.
You can do the marriage contract now, and delay consummation until your first wife gives birth, or you can do the marriage contract and consummate the marriage straight away, because there is no connection between your wife’s pregnancy and your taking a second wife.
Undoubtedly in a case such as yours it is better to do the marriage contract so that you will not do anything haraam. By doing the marriage contract, the woman will become your wife and it will be permissible for you to talk to her, be alone with her and touch her, and everything that it is permissible for a man to do with his wife.
But you have to fear Allaah and treat your wives fairly, and it is not permissible for you to favour one of them at the expense of the other.
And it is not permissible for you to form a relationship with a non-mahram woman. If any such thing has happened then what you must do is repent to Allaah and put an end to this relationship until the marriage contract is done.
And Allaah knows best.