What will happen to mother if dead fetus is not removed right away?

Dead Fetus: Severe infection sets in eventually and the woman can get very ill and even die if left untreated. This would have happened quite commonly in years gone by before the advent of scans and d&cs (operation to scrape the inside of the womb out).

Usually nothing.: A fetal demise is generally not a medical emergency - with notable exceptions: placental abruption, preeclampsia, fever, rash and/or vaginal bleeding. Typically, in the absence of the above, a dead fetus does not cause any ill effects to the mother until it is born, as it is sequestered in its sac. See your obstetrician to discuss the best management in your particular case.