22 weeks! At this point we're coming to the end of the Supreme Court's made-up term of "2nd trimester". By that point the woman feels the baby move and certianly feels a bond. Unless overweight she would already look pregnant so it would no longer be a secret. What could drive a woman to such a decision?
In most cases, financial and/or logistical obstacles to doing it earlier, or late discovery of a serious abnormality. In this case, flat out stupidity ("I'd stopped taking the Pill and my periods were really irregular, so I thought I couldn't get pregnant", then proceeded to ignore other symptoms such as unusual tiredness, until finally going to a doctor at 4.5 months because she "woke up one day and felt quite dodgy").
According to the article, she had just found out she was pregnant at that point. This woman is apparently not the sharpest knife in the drawer.