Fetal Blood Sampling (FBS) for Rh Sensitization During Pregnancy

Exam Overview

Fetal blood sampling (FBS) is the collecting of fetal blood directly from the umbilical cord or fetus. The fetal blood is tested for signs of 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 severe anemia.

If you are Rh-sensitized and you are carrying an Rh-positive fetus, your 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 cells.

What happens

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.

  • The fetus may be given an injection of medicine that temporarily stops movement.
  • A small area of your abdomen is numbed with an injection of local anesthetic.
  • Ultrasound is used to guide a needle through your abdomen into an umbilical vein in the umbilical cord.
  • A small amount of blood is withdrawn into the needle and collected.

You may be given more medicine during FBS, such as antibiotics to prevent infection or medicine to prevent preterm labor (tocolytic drugs).

Why It Is Done

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.)footnote 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 pregnancy.

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 delivered safely.

What To Think About

FBS can be done as early as the 17th week of pregnancy, in the middle of the second trimester.



  1. 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: Saunders Elsevier.


Current as ofSeptember 5, 2018

Author: Healthwise Staff
Medical Review: Sarah A. Marshall, MD - Family Medicine
Adam Husney, MD - Family Medicine
Kathleen Romito, MD - Family Medicine
Kirtly Jones, MD - Obstetrics and Gynecology, Reproductive Endocrinology