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Home > Patients & Visitors > Health Library > Fetal Blood Sampling (FBS) for Rh Sensitization During Pregnancy
Fetal blood sampling (FBS) is the collecting of fetal blood
directly from the umbilical cord or fetus. The fetal blood is tested for signs
anemia and other blood problems. FBS is also known as
cordocentesis or percutaneous umbilical cord blood sampling.
FBS is usually used when a
Doppler ultrasound and/or a series of
amniocentesis tests have first shown moderate to
If you are
Rh-sensitized and you are carrying an Rh-positive
immune system can attack the fetus's red blood cells.
FBS is used to look at a fetus's red blood cell count and oxygen level, and it
also looks for signs that your immune system is destroying fetal red blood
FBS is performed in a hospital's outpatient surgery department. You
will probably be given a
sedative to reduce your and the fetus's movement
during the FBS procedure.
You may be given more medicine during FBS, such as
antibiotics to prevent infection or medicine to prevent preterm labor
If Doppler ultrasound and/or amniocentesis tests show moderate to
severe anemia, FBS is then used. FBS is usually reserved for use after these
tests because of its greater risks—up to 2% of pregnancies miscarry after FBS,
and up to 50% lead to worsened Rh sensitization problems. (This happens after
fetal blood mixes with the mother's blood during the blood sampling.)1
FBS is usually used when a Doppler ultrasound and/or a series of
amniocentesis tests have first shown moderate to severe anemia. Fetal blood
tests show the oxygen level, red blood cell condition, and red blood cell
count. This helps your doctor plan the best treatment for you during your
If the effects of Rh sensitization are severe and the fetus has
severe anemia, a fetal
blood transfusion may be done immediately. Future
transfusions may be scheduled to keep the fetus healthy until it can be
FBS can be done as early as the 17th week of pregnancy, in the
middle of the second
Complete the medical test information form (PDF)medical test information form (PDF)(What is a PDF document?) to help you prepare for this test.
Moise KJ (2009). Hemolytic disease of the fetus and
newborn. In RK Creasy, R Resnik, eds., Creasy and Resnik's Maternal-Fetal Medicine, 6th ed., pp. 477–503. Philadelphia:
September 24, 2013
Sarah Marshall, MD - Family Medicine & William Gilbert, MD - Maternal and Fetal Medicine
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North Kansas City Hospital.