I was not so wrong after all.
Kathryn had a baby all right, but shortly afterwards Abi sleeps with Jason and becomes pregnant as well. They both had girls, and Eleanor orchestrates for the girls to be brought up as twins and Kathryn’s daughters. So Hannah is actually Abi’s child. There are several things that made Kathryn leave Abi behind. The main one was her mother who convinced Kathryn that Abi was a danger as the girl keeps threatening with telling the truth about Hannah. Besides, we know that Kathryn is unstable, and we later learn that she is schizophrenic but afraid for her reputation, her mother hid it from everyone, even Kathryn. And when Kathryn left, she wrote letters to Abi, promising they would be together soon, but Eleanor never gave them to her granddaughter.
The end is quite emotional. Hannah learns about her origins through an article that named her the first baby in the new year. Kathryn breaks down and tells her the truth. In her upset state Hannah has a car accident, and it is at this time that Abi, who is trying to locate the girls, finds her grandmother and therefore her family. I love the interaction between Abi and Hannah, and then the confrontation between Abi and her mother. All this is an eye-opener for Kathryn, who realises she let her mother rule her life, causing pain and grief to her daughters, and she finally acknowledges that she needs help.
The way the novel ends is quite bittersweet. We learn that Adam, who Abi wrote her letters to, returned to her but he died in an accident. However, Abi is not bitter and is even hopeful, because she now has two reasons to look forward to, the two girls.