PRENATAL
DIAGNOSTIC TESTING PROCEDURES
Ultrasound
This
noninvasive imaging modality is used for guidance of invasive prenatal diagnostic
procedures. No adverse effects have been identified when using low-energy
transducers. Current recommendations are to perform sonography in pregnancy
only for obstetric indications. Optimal gestational age for identification of
fetal anatomic structural anomalies is 18—20 weeks. Accuracy of gestational
dating is ±5 days at less than 12 weeks, and ±7 days between 12 and 18 weeks.
Specificity and sensitivity for identification of fetal anomalies is less than
perfect.
Chorionic
Villous Sampling (CVS)
This
refers to aspiration of placental tissue precursors under sonographic guidance
for fetal karyotyping. It is performed between 9 and 12 weeks gestation.
Follow-up testing with triple marker screening and sonogram is needed to assess
for neural tube defects. Placental mosaicism may be rarely present although
fetal karyotype is normal. Pregnancy loss rate is 0.7%. Advantages include the
early gestational age at which karyotype is available.
Amniocentesis
This
refers to trans-abdominal needle withdrawal of amniotic fluid under sonographic
guidance. When performed for genetic purposes (fetal karyotyping,
alpha-fetoprotein determination, biochemical studies), the gestational age is
usually 15—20 weeks. When performed for Rh isoimmunization (bilirubin level),
the gestational age is usually after 24 weeks. When performed for fetal
maturity studies (lecithin-sphingomyelin ratio, phosphatidyl glycerol, the
gestational age is after 34 weeks. Pregnancy loss rate is 0.5%.
Percutaneous
Umbilical Blood Sampling (PUBS)
This
refers to sonographically guided transabdominal aspiration of fetal blood from
the umbilical vein. It is performed after 20 weeks for fetal karyotyping, IgM
antibody detection, blood typing, and intrauterine blood transfusion. Pregnancy
loss rate is 1-2%.
Fetoscopy
This
largely experimental procedure refers to sonographically guided transabdominal
placement of a fiberoptic scope for purposes of fetal tissue biopsy and
coagulation of placental vessels in twin- twin transfusion syndrome. It is performed
between 18 and 20 weeks in suspected congenital ichthyosis. Pregnancy loss rate
is 3—5%.
