USMLE Step 2 CK Lecture Notes 2019: Obstetrics/Gynecology (Kaplan Test Prep)

Figure I-5-2. Follow-up of High MS-AFP

If the true gestational age is less than the assumed gestational age, it would
explain the positive low value. In cases of dating error, repeat the MS-AFP if
the pregnancy is still within the window. A normal MS-AFP will be
reassuring.
If the dates are correct and no explanation is seen on sonogram, perform
amniocentesis for karyotype.