Foetal Blood Sampling

Intrapartum foetal blood sampling takes a small sample of capillary blood from the foetal scalp for pH and lactate. It is used when a cardiotocogram is persistently abnormal and the need for urgent delivery is uncertain.

A normal result is reassuring, and it allows labour to continue. An abnormal result indicates foetal acidaemia and supports urgent delivery. The result reflects the acid base state at the time of sampling, and it can change quickly.

Usage. It is used in labour when the trace is suspicious or pathological and the clinical picture does not justify immediate delivery.

Indications. A suspicious or pathological cardiotocogram in labour, when the trace has not improved with correction of reversible causes.

Preconditions. Cervix dilated at least 3 cm, membranes ruptured, vertex presentation with the scalp accessible, and gestation at or after 34 to 36 weeks (a service threshold).

Preparation and technique. The mother is in the lithotomy or left lateral position. An amnioscope is placed against the foetal scalp, which is cleaned and sometimes coated with a thin layer of gel. A small incision of about 2 mm is made, and blood is collected in a heparinised capillary tube. A volume of about 30 to 50 microlitres is needed for pH or lactate. The sample is analysed within minutes, and the incision site is checked for bleeding.

Contraindications. Maternal HIV infection, hepatitis B or C infection, active genital herpes, foetal bleeding disorders, and face presentation. Preterm gestation under about 34 weeks is a relative contraindication.

Interpretation.

• pH: 7.25 or more is normal. 7.21 to 7.24 is borderline and the sample is repeated within 30 to 60 minutes. 7.20 or under is abnormal.
• Lactate: 4.1 mmol/L or under is normal. 4.2 to 4.8 mmol/L is borderline. 4.9 mmol/L or over is abnormal.
• Lactate measurement needs a smaller volume of blood and has a lower failure rate than pH measurement.

Causes of an abnormal result. Foetal hypoxia from cord compression, uteroplacental insufficiency, uterine hyperstimulation, maternal acidosis, and prolonged labour.

Critical values and findings needing immediate action.

• A pH of 7.20 or under, or lactate of 4.9 mmol/L or over, requires immediate delivery planning.
• A borderline result with an ongoing abnormal trace requires repeat sampling or delivery.

Limitations and artefacts. An inadequate sample, air bubbles, delay before analysis, and contamination with amniotic fluid or maternal blood give false results. Caput succedaneum can alter the result. Sampling can fail in 10 to 20% of attempts.

Complications. Bleeding from the site, infection, scalp haematoma and cerebrospinal fluid leak are uncommon.

Follow up tests. Umbilical cord blood gas analysis at birth, neonatal examination, and coagulation tests in the neonate if bleeding at the site persists.

Special groups. Preterm fetuses and fetuses with suspected coagulopathy are not sampled. Previous sampling does not prevent repeat sampling.