Intrapartum continuous electronic fetal heart rate monitoring using a cardiotocograph (CTG) was introduced in the 1960s to identify and respond to intrapartum fetal hypoxia promptly. Unfortunately, CTG was found to have a high false-positive rate of 60% or more. In addition, no significant decrease in the rates of cerebral palsy or perinatal deaths was reported over a 30-year period despite an increase in caesarean section rate.
Fetal scalp blood sampling (FSBS) was advocated as an ‘additional test’ of fetal wellbeing to reduce the false-positive rate of CTG. The aim was to identify the presence of acidosis (low pH) in a sample of blood taken from the fetal scalp so as to differentiate at least 60% of fetuses who were not hypoxic from 40% of fetuses who were experiencing intrapartum hypoxia when the CTG was classified as ‘pathological’. The aim was to avoid unnecessary operative interventions due to the false-positive rate of CTG.
However, FSBS itself has been shown to have a poor positive predictive value for intrapartum hypoxia and recent systematic reviews have reported no evidence of benefit in reducing the operative interventions.[4, 5] Hence, practising clinicians need to critically examine whether FSBS is a useful test or just a historical ‘outdated’ test perpetuated by an obstetric culture, without sufficient scientific and physiological basis to support its use in modern obstetric practice.
Contrary to the popular belief in the UK, FSBS did not develop as an additional test of fetal wellbeing to reduce the false-positive rate of CTG. FSBS developed as a test of fetal wellbeing in its own right, used by Saling in Berlin, Germany in 1962, before commercial production of CTG machines in 1968. He took scalp blood samples from babies during labour to detect acidosis and published his series. This test, which was then developed as an alternative to CTG, was subsequently introduced in the UK, when CTG was found to have a high false-positive rate, to reduce unnecessary operative interventions.
The recommended ‘normal’ scalp blood pH values were obtained from the original study, which included <80 babies during labour. He recommended that ‘if fetal scalp pH is pre-pathological or even pathological (<7.19), fetal blood sampling should be repeated again and if there is a further fall in pH, then delivery should be instituted’. Hence, there was very limited scientific evidence to support the current National Institute for Health and Care Excellence (NICE) recommendation that stipulates an immediate delivery when the FSBS (scalp pH) value is <7.20.
In the UK, an observational study in 1968 erroneously assumed that a ‘normal’ fetal pH during labour should be between 7.30 and 7.36. Based on this assumption, this author postulated that a pH of <7.25 was suggestive of ‘asphyxia’. A closer scrutiny of this study reveals that ‘clinical evidence of fetal distress’ was made based on the presence of ‘meconium’ in the amniotic fluid and the study had only 37 babies on the ‘fetal distress’ arm.
A subsequent large study that included over 15 000 babies, however, concluded that ‘normal’ fetal pH during labour is skewed to the left, indicating that babies may have a pH of less than 7.20 at birth, even though there is no ‘asphyxia’.
Therefore, current normal and abnormal values for fetal scalp pH that were recommended by the NICE Guideline Development Group on Fetal Monitoring were derived from two small studies[6, 7] that were performed in 1962 and 1968, without sound scientific basis.
A suspicion of hypoxia and acidosis in adults warrants an arterial blood gas analysis to determine pH and other parameters. Clinicians do not even take a venous sample in adults, although this would be technically an easier procedure to perform. This is because it is an arterial sample that reflects oxygenation of the central organs and not a venous or a capillary blood sample.
In clinical practice, no clinician would ever attempt to take a blood sample from a peripheral tissue (fingertips, toes or indeed the adult scalp) to determine acid–base status as it would be considered useless. This is because, in response to hypoxia, release of catecholamines (noradrenaline and adrenaline) would result in intense peripheral vasoconstriction. Resultant diversion of the oxygen from peripheral tissue (fingertips, toes, scalp) to supply central organs would lead to a ‘peripheral acidosis’ in all peripheral tissues. This is a normal compensatory mechanism to cope with hypoxia and therefore, testing a peripheral tissue such as the fetal scalp for acidosis reflects a poor understanding of the physiological response to hypoxia.
Pulse oximetry is used to assess oxygen saturation in adults and based on this practice, fetal scalp pulse oximetry was introduced into clinical practice. However, the results are affected by the presence of meconium and blood and a recent Cochrane Review has concluded that fetal pulse oximetry is not associated with improvement in outcomes.
There are no randomised studies comparing FSBS with a sham or no intervention. Randomised studies on FSBS have only compared lactate to pH testing on FSBS, and therefore the safety and effectiveness of the procedure per se versus no such procedure have not been evaluated by level 1 data.
Fetal scalp blood sampling was introduced in 1962, at a time when there was no other means of monitoring fetal wellbeing in labour, other than a Pinard's stethoscope. Fetal scalp was sampled because it was the only accessible fetal tissue during labour from which blood could be taken for analysis. However, during hypoxia, a fetus diverts oxygenated blood from all its non-essential organs (including the scalp) to perfuse its essential organs (brain, heart and adrenal glands). Hence, it does not make much physiological sense to take a sample from a ‘nonessential organ’ to look for evidence of acidosis during attempts at fetal compensation for ongoing intrapartum hypoxic stress.
Fetal scalp blood sampling looks for changes in pH or lactate in a small sample of blood taken from tissues of the fetal scalp. Therefore, the results are not only influenced by the presence of agents that alter the pH, but also by the site of scalp puncture. There is scientific evidence indicating that taking a scalp blood sample at the site of ‘caput’ (venous oedema on the fetal scalp) gives rise to more acidotic pH values compared with a sample from a normal area of fetal scalp.
In addition, presence of meconium in the amniotic fluid has been shown to significantly reduce the positive predictive value of FSBS for fetal acidosis. This is because meconium contains bile acids that may alter the pH due to contamination. Presence of normal amniotic fluid itself, which is alkaline, has been shown to significantly alter the FSBS result significantly in vivo. In addition, it has been reported that taking a fetal scalp blood sample during a uterine contraction results in more acidotic values than taking scalp blood samples between uterine contractions.
A recent study that compared pH values of paired scalp blood samples concluded that 43% of fetal scalp samples that were obtained from the same fetus at the same time were significantly different and in 16%, this difference crossed the decision threshold for intervention. Therefore, current scientific evidence suggests that FSBS is an unreliable test and its result is influenced by several variables, including contamination with normal amniotic fluid.
One of the great concerns about FSBS is whether a sample of blood that is taken from a peripheral tissue (fetal scalp) accurately reflects oxygenation of central organs. Adamson et al. attempted to demonstrate the correlation between pH of fetal scalp blood sample and pH in the carotid artery and jugular vein in monkeys. Unfortunately, this very small study that included only 11 monkeys and appeared to show such a ‘perfect’ correlation had several methodological flaws. It was not performed during labour but the mother monkeys were anaesthetised before the onset of labour. Hence, it was very likely that, in the absence of re-distribution of blood from fetal scalp to the brain secondary to ongoing hypoxia, a perfect correlation was demonstrated. In addition, the experimental conditions of this study that included only 11 monkeys did not reflect the physiology of human labour as the monkeys were electively anaesthetised. Moreover, fetal scalp oedema (caput) that adversely affects the results of FSBS was also avoided by this procedure and so the results obtained were not a true reflection of fetal scalp pH that would be seen during normal human labour.
Correlation between fetal scalp pH and umbilical arterial cord pH was carried out by Boenisch and Saling on 119 fetuses and had several unrealistic statements including performing a scalp fetal blood sample exactly 15 minutes before a vaginal birth in all fetuses. It is not biologically plausible to predict the exact time of birth in human labour and therefore, it was very unlikely that all fetuses spontaneously delivered exactly 15 minutes after taking scalp blood samples.
Fetal scalp blood sampling does have a historical importance as it was invented by Erich Saling in Germany when CTG machines were not commercially available. However, it was not designed to be an ‘additional test of fetal wellbeing’ as is currently used in the UK. There is very limited, if any, scientific evidence to support its use in modern obstetric practice. American College of Obstetricians and Gynecologists guidelines (ACOG) acknowledged decreased use of FSBS in 2009, its most recent review of the topic. Even the NICE Guideline Development Group, after a detailed review of available evidence, conceded that there was no evidence that FSBS reduced incidence of caesarean sections or instrumental deliveries or influenced long-term outcomes. Despite this, NICE recommended its use in the UK based on ‘clinical experience’ because it was felt that it may reduce ‘some’ operative interventions. However, a Cochrane Systematic Review that was published 1 year after NICE Guidelines confirmed that FSBS did not reduce caesarean section instrumental vaginal birth rates and did not influence any neonatal outcomes.
A recent review on complications of FSBS reported drainage of cerebrospinal fluid as well as several other rare, but potentially very serious, complications, which included haemorrhage and scalp abscess. In my own medico-legal practice, four cases of serious adverse incidents, including a case with hemiplegia that was directly contributed to by FSBS, were encountered within the last 12 months.
Hence, the NICE Guideline Group that recommended the use of FSBS in clinical practice without any scientific evidence but based on the Guideline Group's individual ‘clinical experience’ should urgently review their recommendation in the light of current evidence by Cochrane Systematic Review and reported serious complications. In the meantime, obstetricians should critically review current scientific evidence pertaining to FSBS before performing this test (Table 1).
|Study||Study design||Details of FSBS||Findings||Clinical implications|
Saling et al.
No. of subjects: 69
|Observational study||Scalp blood samples fortaken in 69 babiesduring first andsecond stages oflabour and a normogram wasdetermined||Normal values wereobtained from < 80babies during labour||A very small study todetermine normalvalues.|
Beard et al.
No. of subjects: 37
|Observational study||Presence of meconium orabnormal CTG wasused to make adiagnosis of ‘fetaldistress’. All babieswere delivered by anemergency caesareansection.||This very small study (37babies) erroneouslyassumed that thenormal fetal pH atbirth < 7.25 wasindicative of asphyxia||Current recommendednormal pH value forFSBS (> 7.25) is basedon this erroneousassumption andtherefore, needs to bequestioned.|
Helwig et al.
No. of subjects:15,073
|Prospective observationalstudy||Blood samples werecompared to neonataloutcomes||This large study thatincluded > 15 000babies concluded thatpH < 7.20 was notassociated with poorneonatal outcomes||Current recommendednormal FSBS value(> 7.25) as well asthreshold forintervention (< 7.20)are not based onrobust scientificevidence.|
Odendaal et al.
No. of subjects: 20
|Prospective observationalstudy||Fetal scalp blood wastaken from an area ofcaput as well as onthe ‘non-caput’ areaand the results werecompared||A sample of blood takenfrom an area of caputsignificantly differedfrom a ‘normal’ areaof scalp||FSBS results would beunreliable as thevalues would dependon the area of scalpthat is punctured.|
Carbonne et al.
No. of Subjects: 174
|A prospective multicentreobservational study||Fetal scalp pH wascompared with fetalpulse oximetry duringlabour. Cases withmeconium staining ofamniotic fluid werecompared to thosewith clear amnioticfluid||In the presence ofmeconium staining ofliquor, the positivepredictive value ofFSBS for intrapartumhypoxia was only17%||FSBS is unreliable in casesof a pathological CTGin the presence ofmeconium staining ofliquor.|
Lösch et al.
No. of Subjects 35
|In vitro experimental study||Scalp venous samplewere taken in 35fetuses during labourand were mixed withrespective amnioticfluid samples whichwere diluted up to tentimes in the laboratory||FSBS results are affected by the presence of amniotic fluid as the alkaline amniotic fluid neutralises the acidic pH of the scalp blood.||FSBS may give a falselyreassuring result evenif there is ongoingperipheral tissueacidosis, if the sampleis contaminated byamniotic fluid|
O'Brien et al.
No. of Subjects: 293prospective attempts at FSBS
|Prospective observational study||Consecutive paired fetalscalp blood sampleswere analysed||Analysis of paired scalpblood samplesrevealed that in 43% both fetal scalp samples that wereobtained from thesame fetus at thesame time weresignificantly differentand in 16%, thiscrossed the decisionthreshold forintervention||FSBS results are veryunreliable and show awide variation even iftwo samples are takenin the same fetus atthe same time.|
Schaap et al.
No. of subjects: 37
|Case report and review ofliterature||A case of cerebrospinalfluid leakage afterfetal scalp bloodsampling wasdescribed and aliterature search wasperformed todetermine thecomplications.||Serious complicationsincluding life-threatening haemorrhage, scalpabscess and drainageof cerebrospinal fluidare associated withFSBS||Use of FSBS withoutsound scientificevidence may increasethe risks withoutaffording any clinicalbenefits.|
Fetal scalp blood sampling had a historical importance in obstetric practice, being introduced when CTG machines were not commercially available. However, there is very limited scientific evidence to support its use in modern obstetric practice and although it is generally considered to be a safe test, it is associated with potentially serious complications. Hence, its continued use in modern obstetric practice without level 1 scientific evidence to support its usefulness should be questioned.
In my opinion, we can no longer justify performing FSBS to ‘follow NICE Guidelines’ when such guidelines were not based on sound scientific evidence.
Our reluctance to change our culture and historical practices should no longer be an excuse because our primary objective as clinicians should be to ‘first do no harm’. Continuing to perform FSBS without scientific evidence may result in risks associated with the procedure without any potential benefit, which may lead to adverse clinical and medico-legal consequences.
Disclosure of interests
EC is the co-organiser of the Intrapartum Fetal Surveillance Course at the Royal College of Obstetricians and Gynaecologists and Fetal Monitoring Courses at St George's University of London. He uses fetal ECG for intrapartum fetal heart rate monitoring and conducts several masterclasses on CTG and fetal ECG in the UK and abroad and is a member of the Editorial Board for NHS e-learning on CTG. He is currently involved in revising the international FIGO Guidelines on CTG.
Contribution to authorship
EC is the sole author.
Details of ethics approval
Not applicable to commentary.