Assessing fetal wellbeing

Published on 09/03/2015 by admin

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Chapter 28 Assessing fetal wellbeing

Jackie Chua

The method used in the assessment of fetal wellbeing depends on:

• level of risk to the fetus

• low-risk pregnancy
• high-risk pregnancy

• antenatal or intrapartum monitoring

In a fetus with low-risk status, screening only fetal movements has been shown to identify the fetus at risk of intrauterine death.

Tests available for assessing fetal wellbeing

• fetal movement, including kick charts
• cardiotocography (CTG)
• biophysical assessment
• amniotic fluid volume assessment
• growth
• Doppler flow studies
• fetal scalp pH or lactate

Fetal movement

Antepartum stillbirths account for 50% of the perinatal mortality rate. In late antepartum intrauterine fetal death, 60% occur at less than 37 weeks gestation and 2% at gestation over 42 weeks.

Fetal movement in late pregnancy

• Activity of individual fetuses varies, with only 1% of quiet phases lasting over 45 minutes.
• The perception of fetal movement varies between different women. There is reduced activity in fetuses with malformations.
• There appears to be no advantage to formal counting compared with informal inquiry at antenatal clinic.
• Fetal movement assessment should be used as a screening test to prompt other investigations.

High-risk pregnancies, including hypertensive disorders, vascular diseases, thyroid disease, diabetes, isoimmunisation, multiple pregnancies, as well as reduced fetal movements, vaginal bleeding and prolonged pregnancy, should have regular assessment, including umbilical artery Doppler recordings, fetal growth measurement, amniotic fluid volume, as well as CTG and biophysical profile score.

Cardiotocography (CTG)

A CTG is considered normal when the fetal baseline heart rate is between 120 and 160 beats/minute, the variability of heart rate is between 5 and 25 beats/minute, there are two or more accelerations in a 10-minute period and there are no baseline decelerations. An acceleration of fetal heart rate is a rise in heart rate of ≥15 beats/minute and lasting for 15 seconds or more.

Non-stress test (NST) is a reactive CTG where there are two accelerations exceeding 15 beats/minute amplitude and 15-second duration in 20 minutes.

Some factors that need to be taken into consideration when interpreting a CTG are the gestational age, drugs administered to the mother, the speed of the paper (usually at 1 cm/minute), maternal positioning and fetal abnormalities (e.g. complete heart block).

The variability represents fetal reserve and is an indicator of central, cerebral and myocardial oxygenation. If the period of unreactivity lasts longer than 40 minutes, this is considered a suspicious CTG. If the resting phase is over 120 minutes in duration, the positive predictive value for fetal morbidity/mortality is up to 85%.

False-positive rates of CTG in prediction of fetal hypoxia are up to 20%–40%. False-negative rates are 0.3/1000 for stillbirth and 7/1000 for hypoxia or acidosis. About 10%–15% of CTG recordings are unsatisfactory for interpretation.

Vibroacoustic stimulation has been shown to shorten testing time and the incidence of a non-reassuring CTG in term fetuses. Positive result elicits an acceleration after application.

Biophysical assessment

Biophysical assessment involves the following five parameters with a score of 0 or 2 for each:

• CTG
• fetal breathing movement >30 seconds in 30 minutes
• fetal movements of at least three in 30 minutes
• fetal tone considered normal if prompt return to flexion after extension of limbs/trunk
• amniotic fluid volume: more than one pool of >2 cm in two planes

Table 28.1 lists perinatal mortality rates for biophysical assessment scores.

• false-positive rate 50%
• not shown to improve outcome in high-risk pregnancies
• however, normal biophysical assessment has a good negative predictive value and tends to be reflective of the normal Doppler readings

Table 28.1 Biophysical assessment: perinatal mortality rates

Score Perinatal mortality rate
8–10/10 <1 in 1000 (false-negative 0.9/1000)
6/10 >90 in 1000
4/10 60–600 in 1000

Amniotic fluid volume assessment

• amniotic fluid volume is a reflection of fetal wellbeing or placental health
• different methods of measurement: amniotic fluid index (AFI), deepest vertical pocket (DVP)
• decreased liquor volume is associated with increased perinatal mortality and increased risk of operative delivery
• should be used in conjunction with other methods

Growth

• In high-risk pregnancies, estimated fetal weight can help identify the small-for-gestational-age pregnancies.
• Customised growth charts for ethnicity have been proposed.

Doppler flow studies

• low-risk pregnancies: not shown to reduce mortality or morbidity
• high-risk pregnancies:

• fetal umbilical artery Doppler monitoring reduces perinatal morbidity and mortality
• also can decrease inductions
• the risk increases with the worsening of Doppler flow
• resistance index (RI) thought to be best predictor of outcome
• other blood vessels have been investigated for prediction of adverse pregnancy outcomes, such as uterine artery Doppler, but not currently used routinely
• mainly helpful in timing of delivery

Fetal scalp pH or lactate

• Test in labour when the membranes have ruptured.
• The pH is proportional to pCO2 and lactic acid.
• In the first stage of labour, the mean fetal scalp pH is 7.33.
• A pH >7.25 is regarded as normal.
• A pH of 7.20–7.25 is an equivocal result; repeat in 1 hour.
• A pH <7.20 indicates acidosis; delivery should be undertaken immediately.

Other methods of assessment in labour are fetal pulse oximetry and fetal scalp lactate testing. A lactate reading >4.2 mmol/L may be more sensitive then scalp pH for adverse outcome.

Further reading

Bricker L., Neilson J.P. Routine Doppler ultrasound in pregnancy. Cochrane Database of Systematic Reviews. 2000. Issue 2. Art. No. CD001450. DOI: 10.1002/14651858

Devoe L.D. Antenatal fetal assessment: contraction stress test, nonstress test, vibroacoustic stimulation, amniotic fluid volume, biophysical profile, and modified biophysical profile—an overview. Seminars in Perinatology. 2008;32:247-252.

East C.E., Leader L.R., Colditz P.B., et al. Intrapartum fetal scalp lactate sampling for fetal assessment in the presence of a non-reassuring fetal heart rate trace. Cochrane Database of Systematic Reviews. 2006. Issue 4. Art. No. CD006174. DOI: 10.1002/14651858

Gribbin C., James D. Assessing fetal health. Best Practice and Research Clinical Obstetrics and Gynaecology. 2004;18(3):411-424.

Lalor J.G., Fawole B., Alfirevic Z., Devane D. Biophysical profile for fetal assessment in high risk pregnancies. Cochrane Database of Systematic Reviews.. 2008. Issue 1. Art. No. CD000038. DOI: 10.1002/14651858

Nabhan A.F., Abdelmoula Y.A. Amniotic fluid index versus single deepest vertical pocket as a screening test for preventing adverse pregnancy outcome. Cochrane Database of Systematic Reviews. 2008. Issue 3. Art. No. CD006593. DOI: 10.1002/14651858

Tan K.H., Smyth R.M.D. Fetal vibroacoustic stimulation for facilitation of tests of fetal wellbeing. Cochrane Database of Systematic Reviews. 2001. Issue 1. Art. No. CD002963. DOI: 10.1002/14651858

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