4.2 Fetal Growth Surveillance, Fundal Height & Symphysis-Fundal Height (SFH) Plotting

Fetal Growth Surveillance, Fundal Height & Symphysis-Fundal Height (SFH) Plotting

Fetal growth surveillance is among the most critical responsibilities of the antenatal midwife. Undetected fetal growth restriction (FGR) is the single largest contributing risk factor for avoidable stillbirth, perinatal hypoxic brain injury, and neonatal mortality in the United Kingdom. Conversely, accurate detection of pathological growth failure allows timely multidisciplinary surveillance, Doppler monitoring, and planned delivery to avoid intrauterine demise.

National standards from the Royal College of Obstetricians and Gynaecologists (RCOG Green-top Guideline No. 31) and the NHS England Saving Babies' Lives Care Bundle (SBLCBv3) define the clinical protocols governing symphysis-fundal height measurement, growth chart plotting, and fetal movement assessment.

---

Distinguishing SGA from Fetal Growth Restriction (FGR)

In clinical practice and professional examinations, midwives must distinguish between purely statistical growth parameters and pathological growth failure:

1. Small-for-Gestational-Age (SGA):

   • A statistical definition describing an infant or fetus whose Estimated Fetal Weight (EFW) or Abdominal Circumference (AC) plots below the 10th centile for gestational age on a given reference standard.
   • SGA encompasses three distinct cohorts:
     • Constitutionally small, healthy fetuses (accounting for 50–70% of SGA): Fetuses that are small due to benign parental characteristics (maternal height, weight, parity, ethnicity), demonstrating normal anatomical Doppler velocimetry and preserved growth velocity along their customized centile.
     • Pathologically growth-restricted fetuses (FGR): Fetuses failing to thrive due to placental insufficiency.
     • Non-placental growth restriction: Fetuses with chromosomal aneuploidies, congenital structural malformations, inborn errors of metabolism, or congenital intrauterine infections (e.g. cytomegalovirus, toxoplasmosis).
   • Severe SGA: EFW or AC plotting below the 3rd centile for gestational age, carrying significantly higher risks of perinatal morbidity and mortality.

2. Fetal Growth Restriction (FGR / IUGR):

   • A pathological definition describing a fetus that fails to achieve its genetically determined biological growth potential, almost invariably caused by impaired uteroplacental perfusion, defective trophoblast invasion, and abnormal spiral artery remodelling.
   • A fetus does not necessarily need to be <10th centile to suffer from FGR. For example, a fetus genetically destined to achieve the 90th centile whose growth trajectory falls precipitously to the 15th centile has suffered severe growth restriction despite technically remaining within the "statistically normal" (>10th centile) range.

---

Standardized Technique for Symphysis-Fundal Height (SFH)

In women with uncomplicated singleton pregnancies and no major risk factors for FGR (e.g. BMI <35 kg/m², no large fibroids, no previous SGA), serial clinical measurement of symphysis-fundal height (SFH) is the primary surveillance tool.

Clinical Timing

SFH measurement begins at 24+0 weeks' gestation and is performed at every subsequent antenatal appointment (28, 31, 34, 36, 38, 40, and 41 weeks). Prior to 24 weeks, the uterine fundus lies at or just above the level of the umbilicus (20 weeks), where tape measurement demonstrates poor reproducibility and clinical utility.

Step-by-Step Measurement Protocol

To minimize inter-observer variation and observer bias, midwives must execute the standardized Perinatal Institute / RCOG protocol:

1. Preparation: The woman must empty her bladder immediately prior to examination. A full maternal bladder can elevate the uterine fundus by up to 2–3 cm, creating artificial discordance.
2. Positioning: Position the woman in a comfortable semi-recumbent position with her head supported and knees slightly flexed to relax the rectus abdominis muscles. Ensure a lateral tilt to prevent supine hypotensive syndrome.
3. Abdominal Palpation: Gently palpate the maternal abdomen using warm, flat hands to identify the longitudinal axis of the fetus and determine the highest point of the uterine fundus.
4. Placement of Tape: Secure the zero mark of a flexible, non-elastic tape measure at the highest point of the uterine fundus with one hand.
5. Longitudinal Measurement: Extend the tape measure smoothly along the longitudinal midline axis of the maternal abdomen down to the superior (upper) border of the symphysis pubis.
6. Blind Measurement (Error Reduction):

1. Documentation: The measurement is rounded to the nearest full millimetre/centimetre and immediately plotted on the woman's validated growth chart.

                  Standardized SFH Measurement Technique
                  
  Highest Point of Fundus  ───────────────────────┐
  (Zero mark of tape held here)                  │
                                                 │  Longitudinal midline
                                                 │  axis of gravid uterus
                                                 ▼
  Upper Border of Symphysis Pubis ───────────────┘
  (Metric side FACE DOWN until measured)

---

Customised (GROW) vs. Population Growth Charts

Plotted SFH values and subsequent ultrasound biometry are interpreted against dedicated antenatal growth charts:

Population Charts

Traditional population charts plot fetal growth against a single fixed mathematical standard derived from thousands of unselected pregnancies. The primary limitation of population charts is their failure to account for physiological variables, misclassifying naturally small healthy infants as growth-restricted while missing pathologically restricted fetuses in naturally large parents.

Customised Growth Charts (GROW / Perinatal Institute GAP)

Under the Gestational Network / Growth Assessment Protocol (GAP), widely utilized across NHS maternity trusts, customised growth charts (GROW - Gestational Related Optimal Weight) generate an individualized growth curve tailored to the specific pregnancy. The algorithm adjusts the projected birth weight trajectory for five non-pathological physiological variables:

1. Maternal Height
2. Booking Weight (or early-pregnancy BMI)
3. Parity (parous women have heavier infants than primigravidae)
4. Ethnic Origin (accounts for physiological variations across diverse populations)
5. Fetal Sex (males are statistically slightly heavier than females)

By adjusting for physiological characteristics, customised charts significantly reduce false-positive SGA diagnoses, eliminate unnecessary obstetric scans, and double the antenatal detection rate of true FGR.

---

Referral Indications for Obstetric Ultrasound Biometry

Midwives must recognize explicit referral triggers during routine SFH surveillance. An urgent referral for formal obstetric ultrasound biometry (measuring Biparietal Diameter [BPD], Head Circumference [HC], Abdominal Circumference [AC], and Femur Length [FL] to calculate EFW and amniotic fluid volume) and Umbilical Artery (UA) Doppler is indicated if:

1. Single Measurement Below 10th Centile: The initial or any subsequent SFH measurement plots below the customized or population 10th centile curve.
2. Discrepancy >3 cm Below Gestation: On population charts, a single SFH measurement is more than 3 cm smaller than the gestational age in completed weeks (e.g. an SFH of 28 cm at 33 weeks' gestation).
3. Static Growth (Flat Curve): The SFH measurement shows no interval increase between two successive antenatal appointments separated by 2 to 3 weeks.
4. Decelerating Trajectory: The growth trajectory slows and crosses centile lines downwards (e.g. dropping from the 50th centile at 28 weeks to the 15th centile at 32 weeks).
5. Excessive Fundal Height: The SFH plots above the 90th centile or measures >3 cm larger than gestational age, triggering ultrasound to evaluate for polyhydramnios, fetal macrosomia (EFW >90th centile), or undetected multiple gestation.

---

Doppler Velocimetry in Fetal Growth Restriction

When ultrasound confirms an EFW or AC <10th centile, Doppler velocimetry is used to evaluate the haemodynamic severity of placental vascular resistance:

• Umbilical Artery (UA) Doppler: Directly assesses resistance in the fetoplacental microcirculation:
  • Normal Flow: Low-resistance waveform characterized by high forward diastolic flow.
  • Increased Pulsatility Index (PI): Rising vascular resistance within the placental bed.
  • Absent End-Diastolic Flow (AEDF): Advanced placental disease; end-diastolic flow reaches zero. Associated with high fetal hypoxia; mandates inpatient admission, continuous surveillance, and delivery planning.
  • Reversed End-Diastolic Flow (REDF): Extreme placental vascular obliteration; blood flows backward into the fetal aorta during diastole. Imminent risk of fetal demise; requires immediate multidisciplinary delivery, often via emergency caesarean section after corticosteroid administration.
• Middle Cerebral Artery (MCA) Doppler: Evaluates the "brain-sparing effect". In chronic hypoxia, fetal autoregulation causes cerebral vasodilation, decreasing the MCA pulsatility index to prioritize blood flow to the brain.
• Ductus Venosus (DV) Doppler: Reflects right ventricular afterload and central venous pressure. An absent or reversed a-wave indicates decompensated myocardial acidemia and impending intrauterine death.

---

Fetal Movement Surveillance & Management of RFM

Maternal perception of fetal movements is a vital clinical indicator of fetal neurological integrity and muscular wellbeing.

Chronology of Fetal Movements

• Quickening: Multiparous women typically perceive initial fetal fluttering between 16 and 18 weeks' gestation, whereas primigravidae generally identify movements between 18 and 20 weeks (and almost universally by 24 weeks).
• Established Pattern: A consistent, distinct daily pattern of discrete movement episodes is established between 24 and 28 weeks' gestation.

Debunking Dangerous Clinical Myths

Midwives must proactively dispel outdated and hazardous concepts regarding fetal movements:

RCOG Green-top Guideline No. 57: Reduced Fetal Movements (RFM) Protocol

Reduced Fetal Movements (RFM) is defined as any maternal perception of a significant reduction in the frequency, intensity, or regular pattern of fetal movements. A reduction in fetal movements often precedes intrauterine fetal death by several days as the fetus restricts non-essential muscular activity to conserve oxygen for vital organs.

Immediate Triage Actions

1. Immediate Presentation: Any woman presenting with RFM after 24 weeks must be advised to attend the maternity triage unit or day assessment unit immediately on the same day. Under no circumstances should she be told to wait until the next morning or her next scheduled clinic visit.
2. Handheld Doppler Auscultation: Confirm fetal cardiac activity immediately upon arrival.
3. Cardiotocography (CTG): From 28+0 weeks' gestation, RCOG GTG57 advises a CTG to exclude fetal compromise, and Saving Babies' Lives v3 recommends that it is ideally computerised (Dawes-Redman analysis gives an objective result and can end once criteria are met). Between 24+0 and 27+6 weeks, the fetal heart is auscultated with a handheld Doppler and the woman is reviewed by an obstetrician; many units also use computerised CTG from 26 weeks.
4. SFH & Risk Assessment: Measure and plot SFH; assess for risk factors (hypertension, smoking, pre-eclampsia, advanced maternal age, bleeding).
5. Ultrasound Biometry & Doppler: Formal ultrasound examination (biometry, liquor volume, and umbilical artery Doppler) is arranged (RCOG GTG57: ideally within 24 hours when indicated) if:
   • This is a recurrent episode of RFM after 28+0 weeks (Saving Babies' Lives v3; no scan is needed if a normal growth scan was done in the previous 2 weeks).
   • SFH measurement is abnormal (<10th centile, static, or discrepant).
   • The CTG is abnormal or does not meet computerised criteria.
   • There are persisting maternal concerns or additional maternal risk factors.
6. Discharge Criteria: The woman must never be discharged until a fully normal, reassuring CTG is confirmed, all investigations are reviewed, and clear verbal and written safety netting advice is provided emphasizing that she must return immediately if movements decrease again.