Placental abruption causes 6% of all maternal deaths; in a pregnancy with placental abruption, death of the fetus is 12x more likely than in a normal pregnancy. So, if a doctor tells a young mother that the placenta is detached from her fetus and that the fetus is dangerously underweight (which increases the odds from 12x to about 100x), she should take the advice to abort. That way, she has the chance to have another baby, and her existing children do not lose their mother.
To argue, like Tim Tebow's mom that you should ignore doctors' advice and carry on with the pregnancy any way, is appalling. It worked out for her, but it won't work out for 99 other women.