A 20-year-old G2P1 patient comes to see you at 17 weeks gestational age to review the results of her triple test done 1 week ago. You tell the patient that her maternal serum a-fetoprotein (MSAFP) level is 2.0 multiples of the median (MOM). The patient's obstetrical history consists of a term vaginal delivery 2 years ago without complications. What do you tell your patient regarding how to proceed next?

The multiple marker screening test, also referred to as the expanded AFP test or triple screen, consists of maternal serum measurements of estriol, human chorionic gonadotropin, and a-fetoprotein. The multiple marker screening test is used to determine a pregnant patient's risk of having a baby with an- euploidy and a neural tube defect. The AFP test has the greatest sensitivity when done between 16 and 18 weeks. A maternal serum AFP level that is greater than or equal to 2.0 to 2.5 MOM indicates an elevated risk for a neural tube defect and indicates that further workup and evaluation are needed. The first step when an elevated serum AFP result is obtained is to have the patient undergo an ultrasound to verify that the gestational age of the pregnancy is correct. The sonogram can also identify a fetal death in utero, multiple gestation, or a neural tube or abdominal defect, which could all explain the elevated AFP level. A repeat serum AFP test can be done, because at a level of 2.0 MOM there is some overlap between normal and affected pregnancies. The repeat test should be done as soon as possible; waiting until 20 weeks decreases the sensitivity of the test and wastes valuable time in the workup. An amniocentesis is recommended if a neural tube defect is suspected, in order to measure amniotic fluid levels of AFP and therefore confirm the findings of the maternal serum AFP. The physician would not immediately refer the patient for a chorionic villus sampling because this procedure obtains placental tissue for fetal karyotyping and does not add to information regarding the presence of a neural tube defect. A cordocentesis or percutaneous umbilical cord blood sampling (PUBS) is a procedure whereby the umbilical vein is punctured under ultrasonic guidance and a fetal blood sample is obtained. Usually a PUBS is performed when rapid fetal karyotyping must be done, such as in a situation where severe growth restriction exists. PUBS is most commonly used in situations where fetal hydrops exists to obtain information regarding fetal platelet counts and fetal hematocrits.