Let's put this another way. Do you want to work for an employer that is understanding and supportive with regards to your responsibilities as a father?
If so, be upfront about the baby and if you get a poor response from your prospective employer, then maybe that employer isn't for you.
Quite honestly I find that companies full of young lads run by young lads is not likely to be that supportive, because quite honestly they haven't got a clue what it is like to have such responsibilities, nor do they care (in general).
You are going to be shattered for the next couple of years, surviving on limited sleep. You are going to be able to work your core hours, but don't expect to work overtime without getting ill, or letting your relationship with your partner and child suffer as a result. Whilst the boys are out drinking after work, you are not going to want to be there too.
Congratulations and enjoy the ride. You're going to have this crazy little being that takes your input and outputs crazy shit for the next few years. Children are great fun!