The common reasons are: twin /triplet pregnancy, high risk for down syndrome on dual/triple/quadruple test, recurrent miscarriages, baby not growing properly, previous affected child or suspicious of birth defects.
They usually do it with the help of ultrasound, blood tests, Fetal MRI's. In some cases they might do a needle test like amniocentesis or CVS to take samples of the amniotic fluid/placenta so as to study the baby's chromosomes or know the infection status.
All the pregnant women should see a Fetal Medicine expert irrespective of the normalcy. This will avoid misinterpretation and mis-diagnosis of various conditions of pregnancy. Those with any pregnancy problems abnormalities or fetal growth problems mandate to seek an expert advise from a Fetal Medicine expert.
Ideally, a Fetal Medicine Expert is who has completed a fellowship and has certificates of competence from Fetal Medicine Foundation or an equivalent body. These experts are primarily motivated to minimize the risks to your baby before it is born. They are gynaecologists who have done an advanced degree in screening for fetal defects.
One may argue that birthing is since ages of Adam and Eve, then why do we need a special branch now? The specialised medical discipline- Fetal Medicine is an endeavour to bring a healthy baby to you, diagnosing the abnormalities-structural, chromosomal or growth, before it is born and perhaps treating the fetus in the womb through different medical techniques. The idea behind this is to have a paradigm shift to the management BEFORE the baby is born rather after.
It is a specialised branch of obstetrics, which includes assessing and maintaining the health of the fetus(baby inside the womb),diagnosing illnesses/abnormalities and treating them if possible before the baby is born. Fetal Medicine can be arbitrarily divided into prenatal diagnosis and fetal treatment (fetal surgeries).