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Fetal Blood Sampling (FBS)

Extracted from Myles Textbook for Midwives 15th Edition. Diane M. Fraser, Margaret A. Cooper (Eds). London; Churchill Livingstone: 2009. Courtesy Elsevier.


Access to fetal scope via amnioscope passed through the cervix

The use of fetal blood sampling (FBS) as part of screening for fetal anomalies has declined in recent years because improved molecular and cytogenic techniques all more diagnoses to be made from chorionic villi or amniotic fluid. However, fetal blood may be advantageous when there are ambiguous findings from placental tissue. When there is Rhesus isoimmunisation it may be necessary to determine the fetal haemoglobin, in case intrauterine transfusion is required. Blood can be sampled from the umbilical cord or intrahepatic umbilical vein. In uncomplicated procedures after 20 weeks, FBS is associated with a one per cent risk of miscarriage.

FBS is also used in conjunction with electronic fetal monitoring. When the fetal heart rate pattern is suspicious or pathological and fetal acidosis is suspected, FBS should always be carried out. A blood sample is taken from the fetal scalp via an amnioscope passed through the cervix, and measurement of lactate or pH should be performed. An FBS pH result of ≤ 7.25 (lactate ≤ 4.1) is 'normal' but should be repeated within 30 minutes - 1 hour. An FBS pH <7.20 (lactate ≤ 4.9) indicates that the baby should be delivered.

Further reading

National Institute for Health and Care Excellence Pathways. Fetal blood sampling (part of Intrapartum Care). Available at:

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