No wonder Tracy had behavioural problems if...

... Everyone she knew abandoned her. First her mother dumps her, then in the first episode we find out her foster parents brought her back because the mother (who I assume thought she was infertile) gets pregnant! How did those parents expect Tracy to feel, they treated her as second best to their own child! Fostering a child isn't permanent, but it's not like borrowing a bike, you can't take it back when something better comes along. We see in a flashback that Tracy is upset that she has to go back and says it's not fair, and it's right. What kind of person fosters a child, promises to look after them and gets their hopes up that they'll be living there forever (or at least until they leave for university) and then takes them back because it turns out they CAN have children? "I got pregnant and I thought I couldn't" isn't a valid excuse, and I'm surprised that Jenny didn't reprimand the foster parents more for bringing Tracy back.