Actually, Tina's carrying the child of the man she had a one night stand with - Russ Owen. Tina and Dom were having troubles in their marriage at the time she decided to be Jacqui's surrogate, so he left town and she decided she was going to be Jacqui's surrogate no matter what Dom thought.
When Tina inseminated herself with Tony's sperm, she was already pregnant. This was revealed when she went in for a 12 week check-up and it was confirmed that she was 15 weeks pregnant, and the only man she had slept with in that time period was her one night stand with Russ. When Tina found this out, she told Jacqui. However, Jacqui begged her to keep quiet, which she's done, Jacqui's reasoning was that it would ruin both of their marriages. Jacqui's obviously also desperate to give Tony a child, which she can't do. By this time, Dom had come back, reconciled with Tina, and accepted her choice to be Jacqui's surrogate. He has no idea that the child Tina is carrying is her's and Russ.
Now that Tina might not be able to have children again, this child is her last hope. However, she's already made a promise to Jacqui. The paternity reveal has to come out sooner or later.
I really love how Jacqui and Tony are naming the baby after Max, Tony was like a big brother to Max.