Is there a word in English for a man who leaves his pregnant wife and child?

You may use deadbeat husband, borrowing from deadbeat dad — Dictionary.com

(noun) 1. A father who neglects his responsibilities as a parent, esp. one who does not pay child support to his estranged wife.

You will definitely find many examples for deadbeat husband on Google.

He is a cad and a bounder. These are British English terms, and were very insulting in their day, if spoken by one gentleman to another. They were often used together:

You are a cad, sir, and a bounder, to treat your wife so!

cad, Cambridge English Dictionary

a man who behaves badly or dishonestly, especially to women:

He's a cad and a bounder - I'm not in the least surprised he let you down

The reference says this is British English. I don't know if it is used much nowadays, and it doesn't seem strong enough to me.

bounder, Cambridge English Dictionary

a man who behaves badly or in a way that is not moral, especially in his relationships with women.

Do you need it to be a single word? If so:

abandoner: a person or thing that abandons (Collins)

Example: He hadn't set out to be an abandoner. But when he found himself still incarcerated when the due date came, he became profoundly depressed at the role he was forced to play.

Though not very common in this context, runaway should still work.

ODO:

runaway

NOUN

1 A person who has run away, especially from their family or an institution.

‘How ironic that in this family, the runaway in question is a parent, not some rebellious teenager.’