The Role of the Maternal Fetal Medicine Specialist (part 2 )
Assessment of fetal chromosomal abnormalities can be non-invasive or invasive techniques. The non-invasive methods are ultrasound examination and triple screen marker blood testing. Ultrasound examinations can be classified as a screening or basic (level 1) exam when there is no suspicion of an anomaly and the exam is performed within the expertise of the sonographer. This type of exam would evaluate gestational age, fetal lie (position), volume of amniotic fluid, and location of the placenta. Gestational age can be determined by measuring the standard fetal anatomy, such as biparietal diameter (BPD), head circumference, abdominal circumference, and femur length. The fetal femur length is very useful in predicting estimated gestational age. A comprehensive (level 2) ultrasound examination is performed for patients at risk for having an abnormality and should be performed by an operator with experience and expertise. These exams are generally performed by a maternal-fetal medicine specialist between 16 and 24 weeks gestation. A level 2 examination includes all of the level 1 data plus a detailed anatomical survey of the fetus, particularly an in-depth evaluation of the fetal cardiac, neurologic, and genito-urinary systems. The maternal-fetal medicine specialist would establish the diagnosis, exclude other possibilities, evaluate the prognosis, and make recommendations for further exams or follow-up as appropriate.
The triple screen marker (triple test) blood test involves obtaining a maternal blood sample between 15 and 20 weeks gestation to evaluate potential risk of a fetus with a neural tube defect or other abnormality. This test previously consisted of an assessment of alphafetoprotein, but now adds beta HCG and estriol to the serum alphafetoprotein. This test is often routinely recommended as a screening test for pregnant women, although problems with false positive tests are well described. The triple test screens for various problems in the development of the fetal brain, spinal cord and other tissues of the central nervous system (neural tube), and other developmental and/or genetic disorders. Often this test is the first indication that there may be a problem and therefore skilled interpretation and evaluation of abnormal results are essential.
Invasive techniques include chorionic villus sampling (CVS), amniocentesis, and percutaneous umbilical cord blood sampling (PUBS). In general, the invasive techniques are more accurate than the non-invasive techniques but involve greater risks to the pregnancy. Chorionic villus sampling is an accepted and proven method of prenatal diagnosis requiring genetic counseling, including a detailed discussion regarding the risks and benefits of the procedure. During the chorionic villus sampling test, a small sampling of cells (called chorionic villi) are taken from the placenta where attached to the wall of the uterus. The chorionic villus sampling procedure can be performed transvaginally or transabdominally, depending on the location of the placenta. This procedure is performed between 10 and 12 weeks gestation and results are generally available within 7-10 days. Chromosomal problems such as Down's syndrome or other genetic diseases such as cystic fibrosis, Tay-Sachs disease and sickle cell disease can be diagnosed. Chorionic villus sampling does not detect open neural tube defects and therefore maternal serum triple screen testing is generally recommended. Chorionic villus sampling carries approximately a 1% risk of miscarriage. Other complications may include bleeding, cramping, and rarely infection. Chorionic villus sampling may be appropriate for some pregnant women who require early diagnosis and treatment. Genetic amniocentesis is a standard procedure in which a small amount of amniotic fluid is removed from the sac surrounding the fetus. The sample of amniotic fluid is removed through a fine needle inserted into the uterus through the abdomen under ultrasound guidance. Amniocentesis can also detect certain neural tube defects as well as certain types of birth defects such as Down's syndrome and other chromosomal abnormalities. However, not all birth defects can be detected by amniocentesis but in families with the following significant genetic risks, amniocentesis can be used to diagnose sickle cell disease, cystic fibrosis, muscular dystrophy and Tay Sach's Disease.
Percutaneous umbilical cord blood sampling (PUBS) is another technique in which a sample of blood is obtained from the umbilical cord while the fetus is in utero. This procedure is often used for conditions in which rapid diagnosis or management decisions are crucial. Indications for cordiocentesis include Rh disease with red blood cell isoimmunization, fetal hemolytic disease, hemoglobinopothies, coagulation defects, platelet abnormalities, evaluation of fetal acid-base status, rapid karyotype and intrauterine infection such as rubella, toxoplasmosis and cytomegalo virus. Generally, the umbilical cord can be accessed between 20-30 weeks gestation.
The second subgroup are healthy gravidas with fetuses at markedly increased risk for adverse outcome. Multiple pregnancy or patients with an incompetent cervix are examples in this category. Other conditions would most likely be diagnosed by the obstetrical care provider and not necessarily involve another health care provider. A third subgroup comprises any antepartum patient admitted for "other than delivery" and patients with postpartum complications such as severe hemorrhage, refractory infections, complicated preeclampsia, eclampsia, and difficult post cesarean complications. The fourth subgroup are patients with medical and surgical disorders (Table 3). Any of the patients with the disorders listed would be appropriate for consultation with a maternal-fetal medicine specialist.
In summary, the role of the maternal-fetal medicine specialist can be diverse. As a trained subspecialist with additional experience in the field of complicated obstetrics, the maternal-fetal medicine specialist can be a crucial part of the obstetrical healthcare team. Ultrasound, as a diagnostic and prognostic tool, is often the critical tool of the trade for the maternal-fetal medicine specialist. In addition to ultrasound skills, development of appropriate management plans using a team approach that involves the parents, neonatologist, pediatric subspecialist, genetic specialist, social service counselors and referring obstetrical healthcare provider are essential. As recent advances and new therapies develop in the field, the maternal-fetal medicine specialist should be able to provide insight and skill to these areas. The future of maternal-fetal medicine is bright due to the continued need for answers to clinical problems and complications. Although good outcome data for the use of maternal-fetal medicine specialists is not currently available, it is hopeful that the subspeciality of maternal-fetal medicine will provide convincing data that obstetrical outcomes can be optimized with the use of experts. Both the mother and newborn patients deserve the most skilled expertise available in this highly litigious environment today.
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