If I'm not mistaken, the terms of custody and visitation would be determined by the state in which your wife lives. As she is now in the US and you aren't, I'm afraid the responsibility lies with you to make it known that you want to be involved in your child's life.
What that means is, to a large degree, up to you. If you are on your way to divorce then it depends upon how often you can get to the US to see the child (because obviously, for some time, the child can't come to you!). It is unlikely that you would get any substantial physical custody though you might get some legal custody. You will need a lawyer to help you with this as it's a very complex issue.
If you want to try to save your marriage, get on a plane, get over here, and get into some couples counseling asap. It sounds like you both have been through a hard time so make sure you aren't making a mistake if you possibly can.
Something else you need to be aware of is that most US states are very serious about money (child support). Just because you live in the UK does not make you exempt from this. There is a reciprocity agreement betweent the US and UK and it is enforced - and my understanding is that it is enforced better than the UK enforces it's own child support orders!
You can find out all about child support for the state your wife lives in... just Google "'State' Child Support".
HTH!
