Obstetrics and Gynaecology chapters
Assessment of fetal wellbeing and CTG
How the fetus is checked before and during labour: fetal movements, fundal height and growth scans, amniotic fluid, the CTG and its four features, the biophysical profile, umbilical and fetal Doppler, and fetal blood sampling in labour.
- A normal CTG has a baseline of 110-160 bpm (110-150 at term antenatally), variability of 5 bpm or more, accelerations (15 bpm for 15 seconds) and no decelerations; two accelerations in 20-30 minutes = reactive.
- Late decelerations, reduced variability, tachycardia and a sinusoidal pattern suggest hypoxia; early decelerations mean head compression and variable ones cord compression.
- In a growth-restricted fetus, umbilical artery Doppler guides timing: absent or reversed end-diastolic flow and an abnormal ductus venosus mean danger; in labour a scalp pH below 7.20 confirms compromise.
On this page
Overview
Assessment of fetal wellbeing aims to find the fetus that is short of oxygen (hypoxic) or not growing, so that it can be delivered before damage or death.
| Setting | Main tools |
|---|---|
| Every pregnancy | Fetal movements, symphysis-fundal height from 24 weeks, auscultation of the fetal heart |
| High-risk antenatal | Ultrasound growth scans, amniotic fluid volume, umbilical artery Doppler, CTG, biophysical profile, fetal vessel Doppler |
| Labour | Intermittent auscultation (low risk), continuous CTG (high risk), liquor colour, fetal blood sampling |
Clinical methods
Fetal movements
- Reduced fetal movements are a warning sign of hypoxia or impending stillbirth and need same-day assessment with a CTG (and a scan if risk factors).
- Ask about her baby's usual pattern; a change in pattern matters more than a fixed count.
Fundal height and auscultation
- SFH in cm roughly equals weeks from 20 to 36; smaller than expected → ultrasound (see the antenatal care chapter).
- Listen to the fetal heart with a hand-held Doppler device or a Pinard stethoscope.
Ultrasound
Growth
- Abdominal circumference (AC) below the expected centile is the key sign of a small fetus; serial AC and HC show the growth pattern.
- Estimated fetal weight combines HC, AC, BPD and femur length.
- Asymmetrical FGR (head sparing): HC preserved, AC falls; typical of placental insufficiency (e.g. hypertension).
- Symmetrical FGR: everything small; think of early insults (chromosomal, infection) or a constitutionally small baby.
- In a diabetic pregnancy the AC is disproportionately large.
- Serial scans (plotted on a chart) are needed; growth arrest is ominous.
Amniotic fluid
| Measure | Oligohydramnios | Normal (third trimester) | Polyhydramnios |
|---|---|---|---|
| Deepest vertical pool | Under 2 cm | 2-8 cm | Over 8 cm |
| Amniotic fluid index (sum of 4 quadrants) | Under 5 cm (5-10 cm = reduced) | 10-25 cm | Over 25 cm |
- Low liquor in FGR: blood is diverted away from the kidneys, so less urine.
- Other causes of oligohydramnios: renal agenesis, posterior urethral valves, ruptured membranes, post-term pregnancy.
- Polyhydramnios: fetus cannot swallow (oesophageal atresia, anencephaly), diabetes, twin-to-twin transfusion.
Deepest pools: 1.5 + 1.0 + 1.0 + 0.5 cm
AFI = 4.0 cm → under 5 cm → oligohydramnios
The cardiotocograph (CTG)
How it works
- Two belts: an ultrasound (Doppler) transducer records the fetal heart rate; a tocodynamometer at the fundus records contractions (frequency and duration, not strength).
- Mother lies tilted to the left (or half sitting) so the uterus does not press on the vena cava; record for at least 20-30 minutes.
- A fetal scalp electrode gives a direct signal if the external trace is poor (needs ruptured membranes; avoid with HIV or hepatitis C).
The four features
| Feature | Normal | Abnormal and meaning |
|---|---|---|
| Baseline rate | 110-160 bpm (antenatally at term, 110-150) | Tachycardia: maternal fever or infection, chorioamnionitis, fetal hypoxia, fetal anaemia, beta-agonists (ritodrine), prematurity. Bradycardia: hypoxia, cord compression, maternal hypotension |
| Baseline variability | 5-25 bpm in labour (antenatal traces usually over 10) | Reduced: hypoxia, fetal sleep (up to 20-30 minutes, then recovers), opioids, sedatives, infection, prematurity |
| Accelerations | Rise of 15 bpm or more for 15 seconds or more | Absent: uncertain meaning in labour; a worry antenatally |
| Decelerations | None | Fall of 15 bpm or more for more than 15 seconds; see the types below |
Normal baseline variability shows an intact fetal autonomic nervous system; it is the most reassuring single feature. Accelerations are a positive sign of a non-hypoxic fetus.
The baseline falls with gestation as parasympathetic tone matures, so a preterm fetus may sit near the top of the range. Reduced variability for up to about 30 minutes may be just fetal sleep; it should be followed by normal variability.
Types of deceleration
| Type | Timing with the contraction | Cause | Significance |
|---|---|---|---|
| Early | Begins with the contraction, lowest point at the peak, back by its end (mirror image) | Head compression (vagal) | Benign; common in late first and second stage |
| Variable | Varies in shape and timing; drops and recovers quickly | Umbilical cord compression | Usually benign; worrying if prolonged, deep, or with other abnormal features |
| Late | Begins after the contraction starts, lowest point after the peak, recovers after it ends | Uteroplacental insufficiency → fetal hypoxia | Ominous, especially with reduced variability or tachycardia |
| Prolonged | One long fall in heart rate | Cord prolapse, abruption, uterine rupture, hypotension, hyperstimulation | Emergency if it does not recover |
VEAL CHOP: Variable = Cord compression; Early = Head compression; Acceleration = OK (oxygenated); Late = Placental insufficiency.
A sinusoidal pattern (smooth regular wave with no variability) suggests severe fetal anaemia (rhesus disease, fetomaternal haemorrhage, vasa praevia) or severe hypoxia.
Antenatal CTG (non-stress test)
- Reactive (normal): baseline normal, variability over 10 bpm, two or more accelerations in 20-30 minutes, no decelerations.
- Suspicious: reduced variability, no accelerations, or decelerations. Read in context: with many risk factors, consider delivery; with none, repeat later.
- Computerised CTG measures short-term variation objectively; very low values are linked to acidaemia in growth-restricted fetuses.
Intrapartum CTG classification (NICE approach)
Each of the four features is graded reassuring, non-reassuring or abnormal.
| Category | Definition | Action |
|---|---|---|
| Normal | All four features reassuring | Continue |
| Suspicious | One non-reassuring feature | Correct reversible causes, keep watching |
| Pathological | Two or more non-reassuring, or any one abnormal | Correct causes, VE (exclude cord prolapse, check progress), fetal blood sampling or expedite delivery |
Intrauterine resuscitation: turn to the left lateral position, IV fluids, treat epidural hypotension, stop or reduce oxytocin, give a tocolytic (terbutaline) for hyperstimulation.
Who needs continuous CTG in labour?
- Low-risk labour: intermittent auscultation (every 15 minutes in the first stage, every 5 minutes in the second, for 1 minute after a contraction). A routine admission CTG is not recommended.
- Continuous CTG for: significant meconium, abnormal heart rate on auscultation, maternal fever (38.0°C, or 37.5°C twice), fresh bleeding, oxytocin, epidural, maternal request, and high-risk pregnancies (previous caesarean, FGR, pre-eclampsia, diabetes, preterm, multiple pregnancy).
For a low-risk woman in normal labour, the recommended method is intermittent auscultation, not continuous CTG. CTG has a high false-positive rate; its routine use raised caesarean and instrumental delivery rates.
Biophysical profile (BPP)
An ultrasound assessment over at least 30 minutes (to cover a sleep cycle) with five parts, each scored 2 (normal) or 0:
- Fetal breathing movements
- Gross body movements
- Fetal tone (flexion and extension of limbs or trunk)
- Reactive CTG (non-stress test)
- Amniotic fluid volume (deepest pool of at least 2 cm)
| Score | Meaning |
|---|---|
| 8 or 10 | Normal |
| 6 | Equivocal; repeat within hours (may be fetal sleep) |
| 0, 2 or 4 | Abnormal; consider delivery |
Reactive CTG 2, breathing present 2, tone normal 2, no body movements 0, deepest pool 1.5 cm 0
Total = 2 + 2 + 2 + 0 + 0 = 6/10 → equivocal; repeat
Liquor volume reflects chronic hypoxia; breathing, movements, tone and CTG reflect acute hypoxia. A systematic review did not show BPP lowers perinatal deaths in high-risk pregnancies.
Doppler ultrasound
Umbilical artery (placental resistance)
- Normally there is forward flow in diastole, and diastolic flow increases with gestation as placental resistance falls.
- A failing placenta (infarcts, pre-eclampsia) raises resistance: pulsatility or resistance index rises, then diastolic flow becomes reduced → absent → reversed.
- Absent or reversed end-diastolic flow (AREDF) is strongly linked to hypoxia and intrauterine death; usually triggers delivery planning.
- In high-risk pregnancies (hypertension, FGR) umbilical Doppler reduces perinatal deaths and unnecessary inductions. It is not a screening test for low-risk women.
Fetal vessels (fetal response)
| Vessel | Finding | Meaning |
|---|---|---|
| Middle cerebral artery (MCA) | Low pulsatility (more diastolic flow) | Brain sparing / redistribution in hypoxia |
| MCA peak systolic velocity | Raised | Fetal anaemia (rhesus disease, parvovirus B19, twin-to-twin donor) |
| Fetal aorta | Absent diastolic flow | Fetal acidaemia |
| Ductus venosus | Reduced, absent or reversed a-wave | Heart failure and acidaemia; death imminent |
| Cerebroplacental ratio (MCA ÷ umbilical PI) | Low | Higher risk of intrapartum distress, even in normal-sized fetuses |
Typical sequence of deterioration in early FGR: umbilical artery resistance rises → MCA redistribution → umbilical AREDF → abnormal ductus venosus → abnormal CTG → death. An abnormal ductus venosus or CTG is a late sign.
Uterine artery (maternal side)
- Done at 20-24 weeks in high-risk women (previous pre-eclampsia, chronic hypertension, smoking, diabetes, previous SGA baby or stillbirth).
- High resistance with an early diastolic notch reflects poor trophoblast invasion of the spiral arteries and predicts pre-eclampsia, FGR and abruption.
- A normal result is reassuring; an abnormal one needs serial growth scans and umbilical Doppler.
Fetal assessment in labour
Liquor
- Thick, fresh (bright green) meconium, scanty liquor or heavy blood staining are warning signs. Thin old meconium is common at or after term in a healthy fetus.
Fetal blood sampling (FBS)
- Taken from the fetal scalp through an amnioscope; needs the cervix at least 3 cm dilated.
- Avoid it when the mother carries a blood-borne virus (such as HIV or hepatitis C) or the fetus may have a bleeding disorder.
| Scalp pH | Meaning | Action |
|---|---|---|
| Over 7.25 | Normal | Continue; repeat within 60 minutes if CTG stays abnormal |
| 7.20-7.25 | Borderline | Repeat within 30 minutes |
| Under 7.20 | Abnormal: fetal acidaemia | Deliver (caesarean if not fully dilated; instrumental if fully dilated and safe) |
A base excess worse than −12 mmol/L shows significant metabolic acidosis.
Expect: the normal baseline (110-160 bpm), variability (5-25 bpm) and definition of an acceleration; what makes an antenatal trace reactive; the causes of fetal tachycardia and reduced variability (fever, hypoxia, opioids, fetal sleep); matching early, variable and late decelerations to head compression, cord compression and placental insufficiency; the sinusoidal trace and fetal anaemia; the CTG categories normal, suspicious and pathological; intermittent auscultation for low-risk labour and indications for continuous CTG; BPP components and score meanings; AFI and deepest pool cut-offs; absent or reversed end-diastolic flow; MCA Doppler for anaemia; scalp pH below 7.20; reduced fetal movements needing a CTG.
Quick revision
- Normal CTG: baseline 110-160, variability 5-25, accelerations present, no decelerations.
- Reactive antenatal CTG: 2 or more accelerations (15 bpm, 15 seconds) in 20-30 minutes.
- Early = head compression; variable = cord compression; late = placental insufficiency.
- Sinusoidal trace = severe fetal anaemia.
- BPP: breathing, movements, tone, CTG, liquor; 8-10 normal, 6 equivocal, 4 or less abnormal.
- Oligohydramnios: deepest pool under 2 cm or AFI under 5 cm; polyhydramnios: pool over 8 cm or AFI over 25 cm.
- Umbilical AREDF = danger; MCA PSV raised = anaemia; reversed ductus venosus a-wave = death imminent.
- Scalp pH: over 7.25 normal, 7.20-7.25 borderline, under 7.20 deliver.
- Low-risk labour: intermittent auscultation every 15 minutes (first stage), every 5 minutes (second stage).
Written by The House Job from standard references (Obstetrics by Ten Teachers).
FAQ
Why might a healthy fetus show reduced variability on a CTG?
Fetuses have deep sleep cycles of 20-30 minutes, and opioids or sedatives given to the mother also reduce variability; the trace should recover, so it is extended or repeated.
Why is an early deceleration not worrying?
It is caused by pressure on the fetal head during a contraction, which triggers a vagal slowing that mirrors the contraction and recovers as it ends.
What does absent end-diastolic flow in the umbilical artery tell you?
The placenta is offering very high resistance to fetal blood flow, a late sign of placental failure that carries a high risk of hypoxia and stillbirth.
Which test should be done first for a woman at term with reduced fetal movements?
A CTG on the same day, with an ultrasound for growth and liquor if there are risk factors or the CTG is abnormal.