11/14/24, 10\:44 AM Small for Gestational Age

Small for Gestational Age

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Small-for-gestational age (SGA)\: Estimated fetal weight (EFW) or abdominal circumference (AC) below the 10th centile on
a customised growth chart; severe SGA below the 3rd centile.
Constitutionally small\: 60% of SGA fetuses; growth appropriate for maternal size and ethnicity.
Fetal growth restriction (FGR)\: Pathological restriction of genetic growth potential; may show fetal compromise (e.g.,
abnormal Doppler, reduced amniotic
Non-placental mediated growth restriction\: Due to fetal chromosomal/structural abnormalities, infection, or metabolic
disorders.
Placental mediated growth restriction\: Due to issues with placental implantation/vasculature; includes pre-eclampsia,
autoimmune disease, thrombophilia, renal disease, essential hypertension.
Major risk factors\: Previous stillbirth, previous SGA fetus, cocaine use, maternal age >40, chronic hypertension, renal
impairment, diabetes with vascular disease, antiphospholipid syndrome, threatened miscarriage, low PAPP-A, pre-
eclampsia, cigarette smoking.
Minor risk factors\: Nulliparity, IVF singleton, maternal BMI \<20 or >25, previous pre-eclampsia.
Clinical features\: SGA fetuses detected during routine obstetric assessment; typical
height below 10th centile, low EFW, slow/static growth on fetal growth charts.
Investigations\: Blood pressure and urine dipstick to screen for pre-eclampsia; CTG for fetal heart rate; serological
screening for congenital infections; karyotyping for structural abnormalities; serial ultrasound scans and umbilical artery
Doppler for fetal size and wellbeing assessment.
Management\: Address modi
morbidities (e.g., hypertension, renal disease); antiplatelet agents for high-risk pre-eclampsia; expedited delivery for fetal
compromise.
Complications\: Increased risk of perinatal mortality and morbidity, concentrated in the FGR group; neonatal complications
include prematurity, asphyxia, operative delivery, perinatal death, hypoglycaemia, hypocalcaemia, necrotising enterocolitis;
long-term risks include disability, type 2 diabetes, coronary heart disease.
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A comprehensive topic overview

Introduction

Small-for-gestational age (SGA) refers to an estimated fetal weight (EFW) or abdominal circumference (AC) below the
10th centile on a customised growth chart. Severe SGA refers to an EFW or AC below the 3rd centile.
Around 60% of SGA fetuses are constitutionally small, meaning that fetal growth is appropriate for maternal size and
ethnicity.
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Some fetuses with SGA, particularly severe SGA, will have fetal growth restriction (FGR) as well. FGR implies a
pathological restriction of the genetic growth potential- this can be placental or non-placental mediated growth
restriction. As a result, growth-restricted fetuses may manifest evidence of fetal compromise (e.g. abnormal Doppler
studies, reduced amniotic
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Aetiology

SGA fetuses are divided into the following categories\:
Constitutionally small
Fetal growth restriction (FGR)\: either non-placental mediated growth restriction or placental mediated growth restriction

Constitutionally small

SGA fetuses are constitutionally small, meaning that they have reached their genetic growth potential, in keeping with
maternal size and ethnicity.

Fetal growth restriction

Non-placental mediated growth restriction
Fetuses who have not reached their growth potential, for reasons other than those related to the placenta.
Examples include fetuses with chromosomal or structural abnormalities, fetal infection, or inborn errors of metabolism.
Placental mediated growth restriction
Fetuses who have not reached their growth potential, due to problems with the placenta such as abnormal implantation
and vasculature.
Examples include pre-eclampsia, autoimmune disease, thrombophilia, renal disease and essential hypertension.
Maternal factors can also a
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malnutrition or severe anaemia.

Risk factors

All women should be assessed for SGA risk factors at their booking visit. The presence of risk factors warrants increased
pregnancy surveillance.
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Broadly, SGA risk factors can be divided into major (odds ratio >2.0) and minor (odds ratio \<2.0).

Major risk factors

Major risk factors for SGA include\:
Previous stillbirth
Previous SGA fetus
Cocaine use
Maternal age >40 years
Maternal disease\: chronic hypertension, renal impairment, diabetes with vascular disease, antiphospholipid syndrome
Threatened miscarriage
Low pregnancy-associated plasma protein-A (PAPP-A) – a placental hormone
Pre-eclampsia
Cigarette smoking

Minor risk factors

Minor risk factors for SGA include\:
Nulliparity
I n-v i t r o fertilisation singleton
Maternal BMI \<20 or >25
Previous pre-eclampsia
Any woman with one major risk factor will be referred for serial growth scans, which are essential to monitor and diagnose
SGA.
If a woman has three or more minor risk factors, she should be referred for uterine artery Doppler at 20-24 weeks
gestation. Women with abnormal uterine artery Doppler should also be referred for serial growth scans.
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Clinical features

History

A thorough obstetric history at the booking visit is vital to identify any of the risk factors listed above.
A constitutionally small SGA fetus is unlikely to present with any maternal symptoms.
Pre-eclampsia is a major risk factor for SGA. Women with pre-eclampsia may be asymptomatic (diagnosed on routine
BP/urinalysis) but can present with severe headache, visual disturbances, epigastric pain and sudden onset oedema of
the hands, face, and feet.
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Examination

For patients with no risk factors, SGA fetuses are usually detected during routine obstetric assessment.
Typical
Symphysis-fundal height below the 10th centile
Low estimated fetal weight - serial measurements suggesting slow or static growth (Figure 1)
Figure 1. Fetal growth charts. Fetus A is
growing along the 50th centile. Fetus B is
growing below the 5th centile and is SGA.
Fetus C is showing static growth as
demonstrated by crossing the centile
lines.

Investigations

Bedside investigations

Relevant bedside investigations, performed on every pregnant woman, include\:
Blood pressure\: to screen for pre-eclampsia
Urine dipstick\: to screen for pre-eclampsia
Cardiotocography (CTG)\: if fetal compromise is suspected, CTG can be used to assess fetal heart rate, from which the
degree of fetal hypoxia can be inferred

Laboratory investigations

Additional laboratory investigations which may be appropriate for severe SGA, include\:
Serological screening for congenital cytomegalovirus and toxoplasmosis infection
Karyotyping for fetuses with structural abnormalities; amniocentesis antenatally
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Ultrasound

Serial ultrasound scans
Serial ultrasound scans (USS) are the key investigative tool for diagnosis and surveillance of SGA fetuses, performed from
26-28 weeks gestation. For fetuses at risk of SGA, USS measurement of size and assessment of wellbeing with umbilical
artery Doppler is performed every 3-4 weeks until delivery.
Fetal biometric measurements are plotted on a customised growth chart
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Umbilical artery Doppler
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Umbilical artery Doppler is used to assess fetal wellbeing. Table 1 shows examples of normal and abnormal umbilical
Doppler waveforms. If there is a normal Doppler it should be repeated in at least 14 days. An abnormal Doppler should be
repeated more frequently, and expediting delivery should be considered.
Table 1.Doppler waveforms in a fetus with suspected SGA
Doppler
waveform
Example Explanation
Normal
Normal Doppler with end-diastolic
indicates that there is positive blood
to the fetus during maternal systole and
diastole.
A normal Doppler waveform is
reassuring.
Absent end-
diastolic
Reverse end-
diastolic
Absent end-diastolic
positive blood
systole only, due to increased resistance
in the placental circulation.
Absent end-diastolic
with a higher risk of adverse fetal
outcomes.
Reverse end-diastolic
during maternal diastole there is a
back
circulation, due to increased resistance of
the fetal circulation – this results in fetal
hypoxia.
Reverse end-diastolic
with the highest risk of adverse
outcomes for fetuses.
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Middle cerebral artery Doppler and ductus venosus Doppler can also be used to help time delivery of the SGA fetus. If
these are abnormal then some degree of harm may have already occurred to the fetus, and delivery should be planned
immediately, often by caesarean section as the fetus will not tolerate any potential hypoxia in labour.

Diagnosis

The diagnosis of SGA is made from USS assessment of EFW or AC\:
SGA = EFW OR AC is less than the 10th centile on a customised growth chart
Severe SGA =EFW OR AC less than the 3rd centile on a customised growth chart

Management

Conservative management

Managing modi
Smoking cessation
Drug counselling

Medical management

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Women with co-morbidities (e.g. hypertension, renal disease) should be medically optimised before and during pregnancy
to reduce the risk of SGA.
In women at high risk of pre-eclampsia, antiplatelet agents (e.g. aspirin 150mg daily) should be commenced at 12 weeks
gestation until the birth of the baby.

Delivery

Expedited delivery must be considered for an SGA fetus, with evidence of fetal compromise (Table 2). Delivery may be
induced either vaginally or by C-section, depending on other features of the pregnancy.
Table 2. Recommended mode of delivery for an SGA fetus.
1
Gestation Umbilical artery Doppler Mode of delivery
Recommend Caesarean
section
\<37 weeks Absent/reversed EDF
≤37 weeks
Abnormal O
At 37 weeks
Normal O
Women with a SGA fetus between 24+0 and 35+6 weeks of gestation, where delivery is being contemplated, should also
receive a single course of antenatal corticosteroids.
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Complications

Structurally normal SGA fetuses are at increased risk of perinatal mortality and morbidity, however, the majority of adverse
outcomes are concentrated in the FGR group.
Neonatal complications of FGR include\:
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Iatrogenic prematurity
Antenatal or intrapartum asphyxia
Operative delivery
Perinatal death including stillbirth
Neonatal hypoglycaemia and hypocalcaemia
Necrotising enterocolitis
After the neonatal period, babies born with FGR are more likely to su
also more likely to su

References

Royal College of Obstetricians & Gynaecologists. F e t u s . Published in 2013. Available from\: [LINK]
T h e I n v e s t i g a t i o n a n d M a n a g e m e n t o f t h e S m a l l -f o r-G e s t a t i o n a l -A ge
National Institute for Health and Care Excellence. H y p e r t e n s i o n i n p r e g n a n c y . Published in 2020. Available from\: [LINK]
Royal College of Obstetricians & Gynaecologists. A m n i o c e n t e s i s a n d C h o r i o n i c V i l l u s S a m p l i n g . Published in 2010. Available
from\: [LINK]
Perinatal institute. G r o w t h A s s e s s m e n t P r o t o c o l C a r e P a t h w a y . Published in 2020. Available from\: [LINK]
The Cochrane database of systematic reviews. A n t e n a t a l c o r t i c o s t e r o i d s f o r a c c e l e r a t i n g f e t a l l u n g m a t u r a t i o n f o r w o m e n
a t r i s k o f p r e t e r m b i r t h . Published in 2020. Available from\: [LINK]
https\://app.geekymedics.com/notebook/2653/ 5/611/14/24, 10\:44 AM Small for Gestational Age
Clinical Obstetrics and Gynaecology. F e t a l G r o w t h a n d S u r v e i l l a n c e . Published in 2019. Available from\: [LINK]

Reviewer

Miss Karen Moores
Consultant Obstetrician

Related notes

Amniotic Fluid Embolism
Antenatal Screening for Down’s Syndrome
Antepartum Haemorrhage (APH)
Breech Presentation
Caesarean Section

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Contents

Introduction
Aetiology
Risk factors
Clinical features
Investigations
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