Fetal blood sample (FBS) refers to obtaining a sample of blood from the presenting part of the baby in utero, during labour.  It is used to measure fetal pH or lactate as a way of identifying hypoxaemia and acidosis and stratifying those who require urgent delivery.  

Acidosis reflects fetal hypoxaemia as when hypoxic, fetal metabolism changes from aerobic to anaerobic which results in the production of lactic acid.  This leads to a subsequent drop in fetal pH and provides a measure of the degree of hypoxaemia for the baby in labour.

The indications for FBS include:

• pathological CTG in labour (cervix dilated >3 cm, membranes ruptured)

Before you start an FBS procedure, you must:

• Start conservative measures and offer digital fetal scalp stimulation. Only continue with fetal blood sampling if the CTG trace remains pathological.
  
  
• Confirm the position and dilatation of the cervix (>3 cm), station of the presenting part and ensure membranes are ruptured.
  
  
• Explain the procedure to the woman and obtain her verbal consent (see below).
  
  
• Ensure that the instruments are to hand and that the blood gas analyser is functioning.
  
  
• Ensure there are no contraindications for the procedure.

The contraindications include:

• An acute event, for example, cord prolapse, suspected placental abruption or suspected uterine rupture.
• Acute fetal compromise as suggested by a prolonged ongoing fetal bradycardia of >3 minutes.
  
  
• Risk of maternal- fetal infection transfer, for example maternal HIV, maternal hepatitis viruses or active herpes simplex virus.
  
  
• Fetal bleeding disorders or suspected fetal bleeding disorders, for example haemophilia, maternal thrombocytopenia.
  
  
• Prematurity, less than 34 weeks gestation (i.e. <34+0 weeks).
  
  
• Face presentation.
  
  
• The whole clinical picture indicates that the birth should be expedited, for example maternal sepsis, unstable pre-eclampsia.
  
  
• Do not take a sample immediately following a prolonged deceleration.

Be aware that for women with sepsis or significant meconium, fetal blood sampling results may be falsely reassuring. 

NICE guidelines 2017 recommend that when considering fetal blood sampling, explain the following to the woman and her birth companion(s):

• Why the test is being considered and other options available, including the risks, benefits and limitations of each.
• The blood sample will be used to measure the level of acid in the baby's blood, which may help to show how well the baby is coping with labour.
  
  
• The procedure will require her to have a vaginal examination using a device similar to a speculum.
  
  
• A sample of blood will be taken from the baby's head by making a small scratch on the baby's scalp. This will heal quickly after birth, but there is a small risk of infection.
  
  
• What the different outcomes of the test may be (normal, borderline and abnormal) and the actions that will follow each result.
  
  
• If a FBS cannot be obtained but there are fetal heart rate accelerations in response to the procedure, this is encouraging and in these circumstances expediting the birth may not be necessary.
  
  
• If a FBS cannot be obtained and the cardiotocograph trace has not improved, expediting the birth will be advised.
  
  
• A caesarean section or instrumental birth (forceps or ventouse) may be advised, depending on the results of the procedure.

The sterile FBS pack contains as standard the following:

Sponge holder,  Amnioscope with attachable light source, 2 blades with blade holder, 5 capillary tubes and capillary tube holder, petroleum jelly, 6 large cotton swabs and 5 square green swabs.

In addition you will need

• Lubricant gel –do not use Hibitane as this may alter the pH results
• Ethyl chloride spray
• Water for washing
• Sterile gloves
• Apron

• Ensure Labour Ward co-ordinator is aware you are undertaking this test prior to starting and ensure the blood gas analyser is ready to be used.
  
  
• Place the woman either in a lithotomy or left lateral position with her right leg supported and abducted. Then drape the area around the perineum to provide a clean field.
  
  
• Introduce the lubricated amnioscope. Direct the amnioscope posteriorly and sweep it anteriorly to catch the anterior lip of the cervix.
  
  
• Remove the amnioscope’s obturator. Ideally, the cervix should not be visible. Try to visualise the fetal scalp clearly.
  
  
• Clean the fetal scalp with a cotton wool swab, contamination with liquor or meconium can affect results. Spray the scalp with ethyl chloride to produce a reactive hyperaemia.
  
  
• Apply a thin film of petroleum jelly with one of the large cotton wool swabs to increase surface tension on the fetal scalp; this encourages the formation of droplets of blood.
  
  
• Insert the blade provided in the pack into the scalp to the full depth of the guard. Do not stroke the blade across the scalp as this may produce a lesion that is too large.
  
  
• Collect the blood droplet into your capillary tube. At least two samples should be obtained. The second may be taken while the first is being analysed by an assistant.
  
  
• Apply pressure to the fetal scalp with cotton wool swab at the end of the procedure if any bleeding is evident.
  
  
• Reposition the mother comfortably and explain the results to her with your action plan.

A flow diagram for the interpretation of both pH and lactate is included at the end of this document.

pH rather than lactate is currently used within GGC for interpretation of fetal blood sampling, but please make sure you adhere to your local hospital policy.

The results should be interpreted taking into account the previous pH or lactate measurement, the rate of progress in labour and the clinical features of the woman and baby.

Where a FBS is indicated but a sample cannot be obtained and there is no improvement in the cardiotocograph trace, advise the woman that the birth should be expedited.

When planning to repeat an FBS, the time taken to obtain a sample must be taken into account

If FBS is undertaken during labour, ensure paired cord gases are obtained at delivery and documented in the notes.

Fetal blood sampling flowchart