THE RATIONALE AND TECHNIQUE OF FOETAL BLOOD SAMPLING AND AMNIOSCOPY

Traditional Prenatal Diagnosis: Past to Present

In the nearly 60 years since prenatal diagnosis for genetic disease was first offered, the field of prenatal diagnosis has progressed far past rudimentary uterine puncture to provide fetal material to assess gender and interpret risk. Concurrent with the improvements in invasive fetal sampling came technological advances in cytogenetics and molecular biology that widened both the scope of genetic disorders that could be diagnosed and also the resolution at which the human genome could be interrogated. Nowadays, routine blood work available to all pregnant women can determine the risk for common chromosome abnormalities; chorionic villus sampling (CVS) and amniocentesis can be used to diagnose nearly all conditions with a known genetic cause; and the genome and/or exome of a fetus with multiple anomalies can be sequenced in an attempt to determine the underlying etiology. This chapter will discuss some of the major advances in prenatal sampling and prenatal diagnostic laboratory techniques that have occurred over the past six decades.

Is intrapartum fetal blood sampling a gold standard diagnostic tool for fetal distress?

Developed in 1960s, cardiotocography is a screening test and fetal blood sampling (FBS) is an adjunctive, diagnostic technique to detect fetal hypoxia. A fetal blood sample pH value of less than 7.20 has a higher specificity than a pathological CTG to predict low Apgar score at 1 min. Though with a pathological CTG and despite a normal FBS pH value the risk of delivering a hypoxic infant is 30-50%, FBS has assumed considerable importance in purportedly reducing unnecessary obstetric intervention. The evidence for this is weak: the use of FBS with CTG has been shown to reduce operative vaginal deliveries though not Caesarean sections due to fetal distress. There is no difference in the umbilical artery pH at delivery with the use of intermittent FBS with CTG compared to CTG alone. FBS is an invasive procedure: obtaining an adequate blood sample is often difficult and the pH results are affected by handling of the sample, aerobic contamination and processing. Validation of intrapartum FBS requires that the pH and other values obtained are compared to a 'gold standard' technique. Although FBS has been compared to other tests such as scalp lactate, pulse oximetry, fetal ECG waveform analysis, and central haemodynamics in labouring rhesus monkeys, none of these can be considered as 'gold standard'. In the light of the existing evidence, the role of intrapartum FBS as a gold standard diagnostic technique is unproven.

Impact of fetal blood sampling on vaginal delivery and neonatal outcome in deliveries complicated by pathologic fetal heart rate: a population based cohort study

OBJECTIVE

To compare the impact of electronic fetal monitoring (EFM) alone vs. EFM with additional fetal blood sampling (FBS) in vaginal deliveries complicated by pathologic fetal heart rate (FHR).

METHODS

All deliveries in Hesse between 1990 and 2000 were evaluated for participation in this study. Inclusion criteria comprised (1) pathologic fetal heart rate, (2) singleton pregnancy, (3) cephalic presentation, (4) vaginal delivery, and (5) gestational age at delivery of more than 35 weeks' gestation. In order to analyze the meaning of additional risk factors at birth for the effectiveness of FBS two subgroups were selected depending on the presence of additional risk factors at birth. To examine the impact of FBS in deliveries with pathologic FHR on the mode of delivery and on neonatal outcome, univariate regression analysis was performed and odds ratios (OR) and their corresponding 95% confidence intervals (95% CI) were calculated.

RESULTS

The study population comprised 49,560 deliveries, among deliveries complicated by pathologic FHR, 26% underwent FBS. Deliveries with pathologic FHR and controlled by FBS, with no additional antepartum risk factors, were associated with an increase in spontaneous births OR 1.41 (95% CI 1.27-1.58), and in the presence of additional risk factors OR 1.24 (1.19-1.30). Short-term neonatal outcome parameters were characterized by a lower frequency of severe fetal acidosis (umbilical artery pH <7.0) OR 0.55 (0.42-0.72), and Apgar score <5 after 5 min, OR 0.71 (0.55-0.90).

CONCLUSION

In vaginal deliveries with pathologic FHR the use of FBS as an additional means of intrapartum fetal surveillance is associated with less vaginal operative deliveries, and with an improved short-term neonatal outcome.

Foetal acid-base status in clinical foetal distress and high risk cases

The acid-base balance in 307 Chinese patients in labour was studied in Tsan Yuk Hospital, Hong Kong over a 15-month period. Fifty-six of these were normal cases, 200 cases exhibited clinical signs of foetal distress and the other fifty-one were high risk cases. The results showed that cases of clinical foetal distress had a significantly lower foetal pH than that in the normal control group. Of the different signs of clinical foetal distress, meconium was the least important and foetal tachycardia appeared to be more significant than the others. Cases of postmaturity and uterine inertia also gave a lower foetal pH than normal. It is concluded that foetal blood sampling is indicated in these types of cases.

Foetal blood sampling. Practical approach to management of foetal distress

The practical application of foetal blood sampling in the routine management of patients in labour has been reviewed in a six-month survey, during which time 1,668 patients were delivered at Queen Charlotte's Hospital.Foetal acidaemia (pH 7.25 or less) occurred in 45 of the 295 patients who showed clinical signs of foetal distress. Foetal tachycardia was the presenting sign in 33 of these 45 patients, underlining the importance of this physical sign. Foetal acidaemia in association with clinical foetal distress occurred twice as often in patients who had complications of pregnancy and who were therefore regarded as obstetrically "at risk" as it did in patients who were obstetrically "normal" No cases of acidaemia were detected in any of the foetal blood samples performed routinely on "at-risk" patients in the absence of clinical foetal distress.

Foetal blood sampling in the regional hospital

Several difficulties arise in the introduction of foetal blood sampling in a regional hospital. Ideally there should be a unit sufficient to provide continuous registrar cover (anaesthetic cover and medical cover) of the labour suite. In our hospital duties have been reallocated in an attempt to attain this standard. Both consultant and registrar staff must take adequate study leave to understand the principle and practice of blood sampling. Regular lectures and demonstrations must be given to nursing and resident staff. The cost of the initial equipment is abut pound1,000.double daggerFoetal blood sampling has been employed in the unit since January 1968. Its principal use has been the assessment of "foetal distress" Except for one case no low pH value has been found in a "high risk" patient unless the foetus showed signs of clinical distress.