A friend close to the couple revealed:

“Miley thought she was expecting and wanted to announce it to the world, even before she went to the doctor to verify the results. Liam begged her not to go public yet. He was emotionally out of the romance for months by that point, though he was still sleeping with her. She agreed to stay quiet for Liam’s sake.”

The source went on to explain that Liam allegedly felt trapped by the situation, but ultimately agreed to stay with her:

“[Miley] essentially trapped Liam in the relationship, telling him that if he left her alone, she’d tell the world that he had abandoned her at such a delicate time. She begged him to stay and said that he could have his space, as long as he didn’t break up with her. So Liam obliged. He got his freedom and she got the right to say they were still engaged.”