Chapter 4. Fetal Growth I: Definitions and Fetal Growth Restriction

This chapter covers general principles of fetal growth and fetal growth restriction (FGR). Along with determining gestational age, monitoring fetal growth is essential. Biometry helps detect both growth-restricted and accelerated-growth fetuses. Key terms for growth abnormalities will also be clarified.

Objectives

By the completion of this Section the participant will be able to:

Describe the impact of FGR (or IUGR, intrauterine growth restriction) on labor management and neonatal outcomes

Describe formulas and critical cutoffs for determining IUGR

Describe how labor management may be altered based on fetal growth measurements

Describe the difference between IUGR and SGA

Why is monitoring fetal growth important?

Figure 1. Reasons for monitoring fetal growth

Fetal growth is dependent on the normal uterine environment and provision of nutrients and oxygen through the placenta from the mother. Growth is determined not only by genetic potential, but by environmental potential as well.

Disruption of the utero-placental environment has a direct impact on how the fetus grows, and causes the fetus to adapt to the changes, subsequently affecting growth¹.

Impaired fetal growth increases the risk of perinatal mortality, morbidity, and stillbirth, with a 30% incidence linked to FGR in late third trimester. Growth abnormalities are associated with long-term negative outcomes, including prematurity-related conditions, poorer neurodevelopment, and a higher risk of chronic diseases in adulthood.¹

Birth weights between the 70th and 90th percentiles are associated with the lowest risk

Figure 2. Definitions of EFW and AGA.

The following are definitions of terms related to fetal growth:

EFW refers to “estimated fetal weight,” which is calculated using ultrasound measurements based on biometric parameters.

AGA stands for “appropriate for gestational age.” The criteria for AGA include:

· Individual biometric measures are consistent with each other

· EFW (estimated fetal weight) aligns with established growth curves (between the 10th and 90th percentiles)

Biometr: a POCUS component?

Should fetal growth evaluation be part of a POCUS exam, or should it be considered a standard diagnostic scan? There is national and international consensus that POCUS needs to include growth evaluation due to the potential implications of missing growth abnormalities. This means that a women’s POCUS provider needs to be able to order and interpret growth assessment, even if the provider does not actually perform the exam.

Growth screening is done to identify small babies in particular, since they have the greatest risk for morbidity and mortality. The next step is to differentiate between healthy/constitutionally small fetuses, and those who are pathologically small.

A 2021 study by Wanyoni et al. found that routine ultrasounds at 36–38 weeks in low-risk pregnancies improved detection of SGA and severe SGA babies compared to standard fundal height measurements.²

Definitions for small babies

This chapter discusses small babies, whether due to genetics or abnormalities.

Figure 3. Definitions used for small babies.

· SGA (small for gestational age): Biometric measurements and estimated fetal weight (EFW) consistently fall below the 10th percentile on growth curves.

· FGR/IUGR (fetal/intrauterine growth restriction): Refers to restricted growth due to pathology, with fetal size usually under the 10th percentile. Some organizations define FGR as below the 5th, 3rd centiles, or -2 SD (2 standard deviations below the mean) may also be used. Accurate birth weight prediction is considered possible in severe cases (≤5th percentile).

Perinatal morbidity for small babies

Figure 4. Perinatal morbidity and mortality associated with small babies.

Infants with low birth weight constitute a higher proportion of perinatal morbidity and mortality. Key points include:³

· Fetal growth restriction (FGR) is associated with increased risks of intrauterine fetal demise, neonatal morbidity, and neonatal mortality. This population also experiences a significantly elevated rate of stillbirth; up to 40% of stillborn infants are growth restricted.

· Prediction of birth weight at delivery is most accurate in cases of severe growth restriction (birth weight ≤5th percentile).

· While not all small-for-gestational-age (SGA) newborns have underlying pathology, they are at increased risk for newborn complications such as hypoglycemia, hyperbilirubinemia, hypothermia, intraventricular hemorrhage, necrotizing enterocolitis (NEC), seizures, sepsis, respiratory distress syndrome, and neonatal death.

· Additionally, there is an increased risk of cognitive delay in childhood among these infants, as well as greater incidence of adult-onset conditions including diabetes, obesity, coronary artery disease, and stroke.

Importance of Early Gestation dating

Figure 5. Importance of early gestation dating.

Accurate fetal growth assessment relies on precise determination of gestational age, which increases the ability to identify abnormal growth.

Accurate ultrasound measurements in the first and second trimesters (before 23 weeks) are essential. Earlier gestational age estimates are more precise. Correct crown rump measurement is explained in the First Trimester course but is reviewed in the figure below.

Figure 6. Review: principles of determining gestational age during first and second trimester.

Review: Determining gestationatl age;

Determination of gestational age by ultrasound⁴

Gestational age is determined as follows:

First Trimester:

· Use CRL (via transvaginal or transabdominal ultrasound from at least 7 weeks and up to 84 mm; for larger sizes, use BPD.

· CRL and BPD have similar accuracy between 12–14 weeks.

Second Trimester:

· Base gestational age on multiple biometric parameters, not just one.

· Confirm interval growth with a follow-up ultrasound 2–3 weeks after a third trimester scan is done for the first time in pregnancy.

Estimated fetal weight (EFW)

EFW is used to monitor fetal growth following surveillance guidelines. ⁵ Growth parameters are used by the ultrasound computer to estimate the fetal weight, as described in the previous chapters.

To check for size discrepancies, individual and combined ultrasound biometry parameters are compared with known dating from early pregnancy, or from LMP-based estimates.

Individual parameters are compared with one another, to look for discrepancies between the parameters themselves. For example, a HC of 24 weeks and an AC of 18 weeks indicates a potential issue needing further review. Ultrasound software provides separate percentiles for each biometric measure and combines all measured biometric data to calculate EFW.

Figure 7. EFW to determine size discrepancy.

Percentiles of EFW do not always exactly correlate with the individual biometric measures, since EFW is a composite or averaging of all of them to derive a single weight percentile. For example, a combination of a large head and a small abdomen may result in an average EFW for the infant, while the AC percentile by itself (small percentile) may be a sign of growth restriction.

Disadvantages of EFW measurement on ultrasound include: ⁵

· Errors in single parameters affect the entire estimate of EFW.

· High intra-observer variability reduces accuracy.

· Error rates are typically 10–15%, and even higher in SGA or LGA fetuses.

It may be tempting to repeat a concerning growth ultrasound every week or so to follow trending of the EFW, but ultrasounds for EFW should be spaced at least three weeks apart to avoid inaccuracy. Interval scans for other reasons (such as fetal testing or BPP) are acceptable.

Fetal growth charts

Many ultrasound machines generate fetal growth charts to track growth patterns and identify accelerations or decelerations, which may signal abnormal development. In this example, the fetus is at the 50th percentile at 29 weeks’ gestation but rises to the 90th percentile at term, suggesting large-for-gestational-age status.

Figure 8. Examples of fetal growth charts from ultrasound report.

Comparing growth standards

Blue et al. compared the internationally-recognized Hadlock fetal growth standard with the NICHD (National Institute of Child Health and Human Development) racial/ethnic standard for predicting neonatal morbidity and SGA⁶. The assumption was that there are ethnic differences in fetal sizes, making one standard not applicable for all ethnicities. Ultrasound machines use various standardized growth programs, each offering different calculation packages for biometric measurements and EFW, with some allowing user selection. The Hadlock standard has been a leading reference for 25 years.

Figure 9. Comparing growth standards.

Some growth curve studies include factors such as maternal height, weight, age, parity, fetal sex, and ethnicity. The NICHD developed race/ethnicity-specific standards to address concerns about the Hadlock standard favoring white, American populations. However, Blue et al. found that the Hadlock standard predicted neonatal morbidity and SGA more accurately than the NICHD's specific standards, with no significant differences by race or ethnicity regarding EFW.⁶

ISUOG distinction between SGA and FGR ⁷

SGA fetuses are small but generally not at higher risk for complications.

· FGR fetuses are small and have a greater risk of adverse outcomes.

· Fetuses with birth weight below the 10th percentile have an increased stillbirth risk.

Figure 10. ISUOG definitions of SGA versus FGR.

Early vs Late FGR

FGR is classified as:

· Early onset: before 32 weeks

· Late onset: after 32 weeks

The terms ‘Symmetrical’ and ‘Asymmetrical’ associated with growth restriction have been used in the past but are not considered outdated and not informative for etiology.³ Instead, the terms ‘Early’ or ‘Late’ onset FGR are more appropriate.

Table 1. Comparison of Early versus Late FGR.

Diagnosis and surveillance of late-onset fetal growth restriction (FIGUERA)

This chart compares Early and Late FGR:

· Early FGR affects about 1% of pregnancies, often linked with placental disease (60%, usually with pre-eclampsia). Complications include low cardiac output and high vascular resistance.

· Late FGR appears in roughly 10% of pregnancies, with lower placental disease incidence (15% with pre-eclampsia). Fetal hemodynamics show brain-sparing, normal cardiac output, and vascular resistance.

Clinical characteristics of Early vs Late FGR

Table 2. Comparison of clinical characteristics of Early vs Late FGR.⁷

· Early-onset FGR (<32 weeks): Very small fetus, abnormal BPP, high perinatal mortality.

· Late-onset FGR (>32 weeks): Fetus may not be very small, abnormal BPP possible, low perinatal mortality, gestational hypertensive disease uncommon.

This distinction helps identify babies at the highest risk who need closer monitoring.

Additional definitions

Definitions of FGR vary across organizations, making consensus challenging.^3,7

· ACOG notes inconsistent terminology. FGR includes fetuses with estimated fetal weight (EFW) or abdominal circumference below the 10th percentile for gestational age, while SGA refers to birth weight under the 10th percentile; neither considers individual growth potential.

· ISUOG defines FGR as failure to reach growth potential and distinguishes it from SGA, emphasizing that not all SGA infants are growth-restricted and measurement symmetry is important.

· This means that if a baby is found to have an EFW of 25^th percentile at 24 weeks, and remains at 25^th percentile in 3^rd trimester, this is likely to represent a constitutionally small baby, and not a growth restricted baby.

· This also means that a baby found to be at the 65^th percentile at 24 weeks, then drops to 20^th percentile in 3^rd trimester, this represents a baby with growth restriction.

Each fetus should be compared to itself when looking at growth trends.

Figure 11. Additional definitions of FGR

Etiology of FGR

Several causes are linked to FGR, and are categorized into maternal, fetal and placental etiology. ^3,7
Figure 12. Etiologies of FGR: maternal, fetal, and placental.

Preeclampsia and FGR

This diagram explains how pre-eclampsia is associated with FGR.

In normal pregnancy, trophoblasts break down arterial walls, widening vessels in the decidua and increasing blood flow to the fetus.

In preeclampsia, spiral arteries don't transform properly, as trophoblasts fail to invade the myometrial segment. This keeps the arteries narrow, lowering uteroplacental perfusion and disturbing fetal blood supply.

Figure 13. Illustration of how preeclampsia contributes to FGR.

Fetal growth diagnosis and evaluation

Ultrasound assessment of fetal growth and estimated fetal weight (EFW) helps identify potential underlying diseases. However, EFW accuracy relies on proper measurement and may differ from actual birth weight by up to 20% in 95% of cases, with 5% off by at least 20%. If EFW is below the 10th percentile, further evaluation such as amniotic fluid index (AFI), modified biophysical profile (BPP), or Doppler studies of the umbilical artery is recommended.³

Figure 14. Ultrasound diagnosis and evaluation of EFW.

ISUOG recommendations for biometric measuring

As a review, ISUOG recommends the following guidelines for biometric measurements:⁷

Figure 15. ISUOG recommendations for biometric measuring.

Other recommendations

ACOG recommendations suggest using a simpler approach. If fundal height lags by 3 cm or more during a routine prenatal check, an ultrasound is warranted. Of course, fetal position (such as an oblique lie) can affect accuracy of a fundal height measurement.³

Figure 16. Other recommendations for FGR screening: fundal height measurement.

Management of FGR

After detecting FGR, management recommendations include:^{3, 5, 7}

· Consult or refer for suspected FGR.

· Refer to the relevant unit for tailored management.

· Arrange detailed fetal ultrasound evaluation.

· Monitor pregnancy in tertiary-level fetal medicine and neonatal units according to standard protocols.

· Perform serial ultrasounds every 3–4 weeks to check for progression.

Figure 17. Outline of management for FGR

The BPP (biophysical profile) is a key tool for assessing fetal health and response to changes in circulation. A BPP score of 4 or less needs urgent consultation and consideration of delivery options.^{3, 5}

Details of the BPP are presented in the Second and Third Trimester course.

Figure 18. Comparison of a normal versus abnormal BPP score.

Previous non-medical approaches for treating FGR, such as nutritional supplements, bed rest, and aspirin, have not been proven effective according to ISUOG and ACOG. ^{3, 7}

Figure 19. Ineffective management approaches for FGR.

Antenatal surveillance: Doppler

In addition to the BPP, the Doppler ultrasound assesses blood flow between the fetus and placenta. It measures the amount of 'back-flow' in the cord during diastole, indicating placental resistance. Increased resistance can reduce circulation to the fetus and may lead to distress. ^{3, 7}

Figure 20. Image 1. Example of Doppler study of the umbilical artery.

Antenatal surveillance: electronic fetal monitoring (EFM) or Cardiotocography (CTG)

Other antenatal surveillance methods include fetal monitoring via non-stress tests (monitoring fetal heart rate variability) or stress tests (assessing heart rate response to induced contractions).^{3, 7}

Figure 21. Use of EFM or CTG to monitor fetal heart variability and reactivity (a non-stress test or NST).

Management of Early FGR

Delivery management for FGR follows these principles: ^{3, 5, 7}

· FGR alone does not warrant cesarean delivery.

· Use antenatal corticosteroids if birth is expected before 33 6/7 weeks (sometimes up to 35 6/7 weeks).

· Consider magnesium sulfate before 32 weeks for fetal and neonatal neuroprotection.

· Management depends on cause, gestational age, and surveillance findings.

Figure 22. Management guidelines for delivery in early FGR.

Timing of delivery

Delivery timing depends on FGR severity:^{3, 5, 7}

· For isolated FGR, deliver between 38 0/7 to 39 6/7 weeks.

· With additional risk factors (e.g., oligohydramnios, abnormal Dopplers, maternal risks), consider delivery from 34 0/7 to 37 6/7 weeks.

In cases of extreme prematurity (<27 weeks or <600 g), each extra day of gestation increases survival by about 2% but must be balanced against fetal compromise.

Regional guidelines may vary but it a general recommendation is delivery by 37 weeks and no later than 39–40 weeks, with ongoing monitoring (BPP and fetal monitoring) after 37 weeks.

Figure 23. Delivery timing guidelines for early FGR.

Late FGR

Diagnosis and surveillance of late-onset FGR is outlined in Figure 24 (below).⁸

Figure 24. Late FGR etiology, risks, screening.

Late FGR (after 32 weeks) presents distinct challenges and management strategies. Its pathophysiology differs from early FGR but may involve placental lesions and impaired oxygen and nutrient diffusion. While late FGR is only mildly linked to hypoxic events, UA (umbilical artery) Doppler changes are uncommon and often miss most cases. Although poor perinatal outcomes can occur, they are generally less severe than with early FGR.

Late FGR fetuses near term do have reduced tolerance to hypoxemia.

Uteroplacental vascular lesions (especially infarcts) can be noted

Doppler study alone cannot reliably distinguish between late SGA and early (true) FGR. Inducing labor in late FGR does not improve perinatal outcomes and may increase neonatal admissions if done before 38 weeks.

Late FGR management

Performance of third-trimester ultrasound for the prediction of late onset FGR

Figure 25. Review of Cardadeax study comparing measures of AC versus EFW for predicting SGA.

Cardadeaux et al. (2019) conducted a systematic review and meta-analysis on ultrasound diagnosis of late-onset FGR using EFW versus AC measurements, including 21 studies. They found that third trimester AC and EFW are equally accurate for predicting SGA (small-for-gestational-age) infants, but AC is superior for diagnosing FGR. The study recommends scanning at or after 37 weeks. ⁹

Xx Mgmgt of Late FGR Monitoring

Monitoring to prevent stillbirth (ISUOG ASS, ISUOG 7, ACOG 227)

Figure 26. Monitoring Late FGR with Doppler studies and BPP.

Monitoring UA-PI (umbilical artery pulsatility index) and MCA-PI (mid-cerebral artery pulsatility index) are key Doppler studies used to monitor fetuses at risk. The median time from low MCA-PI to stillbirth is five days or less. Nearly 90% of stillbirths occur within one week of a normal BPP, suggesting limited value in using BPP to guide monitoring frequency. Despite this, ongoing fetal monitoring remains essential, since abnormal BPP and nonstress test results can guide when to intervene and induce labor. This is especially true in regions with limited access to maternal-fetal specialists or advanced ultrasound.

Biophysical abnormalities detected by BPP include altered fetal breathing, reduced AFI, and loss of heart rate reactivity. Notably, BPP only becomes abnormal shortly before stillbirth.

Management of Late FGR – Timing of delivery

Figure 27. Outline of management guidelines for delivery timing for Late FGR per ISUOG

Management of Late FGR – Timing of delivery: Consensus^{3, 5, 7}

There is no international consensus on delivery timing for FGR. ISUOG and ACOG expert opinions recommend:

· For late FGR with UA-PI >95%, consider delivery after 36⁰/₇ weeks but before 37⁶/₇ weeks.

· For early FGR (26–34 weeks), consider delivery if any of the following:

· Unprovoked fetal heart rate decelerations

· BPP ≤4

· Maternal indication

· For 36⁰/₇ to 39⁰/₇ weeks, consider delivery with any of the following:

· Unprovoked fetal heart rate decelerations

· BPP ≤4

· Short term variability <4 beats on CTG/NST

· Absent or low end-diagnostic flow in UA Doppler

· Maternal indication

ISUOG FGR recommendations: Summary

The following summarizes ISUOG recommendations for fetal growth restriction (FGR): ^{5, 7}

Figure 28. ISUOG recommendations: FGR summary.

· Fetal size by itself does not confirm FGR unless abdominal circumference (AC) or estimated fetal weight (EFW) is below the 3rd percentile.

· A decrease in fetal growth velocity greater than 50 percentiles, such as a drop from the 70th to the 20th percentile, indicates possible FGR.

· Doppler assessment of uteroplacental and fetoplacental circulations can differentiate between small for gestational age (SGA) and FGR.

· For suspected FGR, multimodal assessment involves CTG/fetal monitoring or biophysical profile (BPP) combined with doppler evaluation:

ISUOG Recommendatsions: SGA

As presented earlier in this chapter, ISUOG’s definition of SGA is consistent with other organizations’ definitions. It is that the fetus is constitutionally small but otherwise healthy⁷.

Figure 29. ISUOG definitions of SGA.⁷

While these babies may be healthy, potential complications are possible, so increased surveillance is appropriate.

ACOG (American College of Obstetricians and Gynecologists) provide guidelines for assessing fetuses at risk for FGR, included in Figure 30 below.¹⁰

Figure 30. ACOG guidelines for antenatal surveillance of FGR babies.

· Monitor fetal Doppler indices.

· Universal induction at term is recommended over expectant management.

· Deliver after 38 weeks gestation but not later than 39 weeks.

· Use continuous fetal monitoring in labo

SUMMARY

One of the primary reasons for performing ultrasound in pregnancy is to monitor fetal growth and well-being.

Ultrasound is useful in identifying those babies who are at increased risk for stillbirth and neonatal mortality/morbidity.

Fetal growth restriction is categorized by either Early FGR (less than 32 wks.) or Late FGR (later than 32 weeks.

The earlier the FGR, the more severe pathology in most cases.

Surveillance of any suspected growth abnormality must be consistent with guidelines, and a consult/referral network must be utilized by the midwife or obstetric provider if there are significant variations in biometry.

Doppler velocimetry is useful in determining how placental perfusion may be contributing to growth restriction, and this should be made available to those who have access to this level of ultrasound exam.

Large babies are also at increased risk for neonatal morbidity and mortality, as are mothers who give birth to large babies.

Early recognition is necessary to more carefully monitor fetal growth patterns.

Appropriate delivery timing should be offered to those at risk for significant FGR.

Women's Point-of-Care Ultrasound: Fetal Biometry and Growth

Chapter 1. Fetal Biometry and Viability: Overview