You're still married. If you want a divorce you could file for divorce, but if you still want to make him legal then you'd have to go through the regular procedures. Talk to either an Immigration lawyer or a divorce lawyer.
Deported
If the child is a legal citizen they go to the closest legal relative, if there is no one to take the child they go into foster care. If the child is not a legal citizen they will be deported with the parent. If the child is a legal citizen and they aren't registered in school they can be deported with the parent as well.
The CitiZen Chil will be adopted by Federal / State
If the child was born on American soil, the child is a legal citizen. The child then has the option of staying, be it a very poor option.
Assuming youre a UK citizen, you are no longer eligible for the Visa Waiver Program and would require a visa to visit the US. Moreover, the deportation record would make it difficult for you to get said visa, or even get residency in the US based on a marriage to a US citizen. If youre happy with never visiting the US, you should be fine.
She becomes an American citizen.
If a citizen of Russia is attending school in the US on a Student Visa and becames pregrant by an American what happens to her Student Visa?
If you are a foreigner marrying a US citizen, then no you are not a US citizen. Marriage to a citizen does normally give you "permanent resident" status for the duration of the union, and qualifies you for certain paths to citizenship, but it's not automatically granted.
I married an illegal immigrant in July, 2003. As a result I cannot foster children or adopt them. I probably will not get custody of my neice that I have applied for. I cannot get health insurance because his income cannot be counted partially because he is not allowed to work, partially because he is paid cash. I worry every night when he is later than usual from work that the immigration has picked him up, I cannot get into student housing because he has no social security number, etc., etc., etc. If I were not so happy in my relationship, I would have ended this marriage to reclaim my rights as an American citizen. Basically, by entering this marriage, I have made myself an illegal immigrant with little to no rights.
Absolutely nothing.
Ju get deported
The child can go with the parents.