When an infant has an estimated fetal weight or abdominal circumference that is below the 10th centile, they are referred to as being small for gestational age (SGA) (University Hospitals Plymouth NHS Trust 2021).

These babies fall into one of two broad categories:

1. Infants who are constitutionally normal
2. Infants who are small for their gestational age due to growth restriction, which results in lower-than-expected birth weight.

(Osuchukwu and Reed 2021)

For those infants who are healthy and constitutionally normal, their low birth weight may simply be the result of factors such as maternal height, weight, ethnicity and parity. In these cases, there is no increased risk of perinatal mortality or morbidity.

However, the outlook is more concerning for babies whose reduced birth weight is caused by fetal growth restriction (FGR). With an overall incidence of about 11% in Australia, the causes of FGR are diverse and may include both fetal and maternal factors, or be linked to demographic influences (Osuchukwu and Reed 2021).

In 2017, Kildea et al. specifically looked at the rates of SGA babies amongst Aboriginal women from remote communities in Northern Australia and discovered that the SGA rate was about 16.3%, suggesting that these women may need a combination of targeted antenatal education along with better access to antenatal care.

Assessing the Rate of Growth

Many SGA babies are first identified upon routine palpation during second and third-trimester antenatal visits when the fundal height appears at least 3 cm lower than expected for the stage of gestation (Osuchukwu and Reed 2021).

Confirmation is then made via ultrasound scan, which can detect a pattern of restricted growth over three or four measurements. Criteria for diagnosis include:

• The first fundal height measurement plotting below the 10th centile on the customised chart
• No growth over two consecutive measurements, with no less than two weeks between measurement
• The plotted measurements falling below the expected growth trajectory based on previous measurements.

(University Hospitals Plymouth NHS Trust 2021)

Small for Gestational Age or Intrauterine Growth Restriction?

The term intrauterine growth restriction (IUGR) is sometimes mistakenly used interchangeably with SGA. However, IUGR is caused by placental insufficiency, which can compromise the growth rate of the fetus, whereas the cause of SGA is more likely to be multifactorial (Cuttler, Misra & Koontz 2016).

Identifying Risk Factors

Of the many studies that have explored the causes of SGA, most identify obesity and smoking during pregnancy as likely causes of restricted growth.

Knight-Agarwal et al. (2020) explored the relative risk reduction of SGA in women who stopped smoking, compared to those who continued smoking throughout their pregnancy.

The results were clear: smoking during pregnancy can inhibit fetal growth. Given the clarity of these results, Knight-Agarwal et al. suggest that antenatal smoking cessation sessions could be a highly effective strategy to reduce preventable infant morbidity and mortality.

The following conditions that can be identified at booking are also considered risk factors for SGA:

• Age ≥ 40 at estimated date of delivery
• Smoking more than 10 cigarettes per day
• Current drug use
• Daily vigorous exercise
• Previous baby ≤5th centile for gestation
• Previous stillbirth
• Chronic hypertension requiring medication
• Diabetes
• Renal impairment
• Antiphospholipid syndrome
• Fibroids >6cm, or multiple fibroids
• BMI ≥ 35
• Bicornuate Uterus.

(University Hospitals Plymouth NHS Trust 2021)

Osuchukwu and Reed (2021) emphasise the importance of an early review of any chronic maternal conditions such as hypertension, renal disease or diabetes mellitus. They also flag up the importance of identifying any prior history of SGA within the mother’s obstetric or family history. Finally, they also acknowledge the role social stressors can also play in sub-optimal fetal growth.

Given that women of low socioeconomic status are known to be more susceptible to poor infant birth outcomes, McRae et al. (2018) explored the possibility that enhanced antenatal care could minimise the risk of adverse outcomes for this population of mothers. Not surprisingly, they found that outcomes improved with increased availability and accessibility to the maternity team, but they also flagged the need for further research to determine which aspects of care were specifically of greatest value. For example, it’s still unclear whether psychological or physical care has the greater impact on positive outcomes.

New Australian Birthweight Centiles

Joseph et al. (2020) raise an interesting issue by suggesting that new birthweight charts are needed, based on data for spontaneous singleton births in Australia. If these changes are implemented, it’s likely that some fetuses previously regarded as normal would be identified as small for gestational age. Although it would increase the burden of health care services if more cases of SGA were diagnosed, it may also have the benefit of reducing the incidence of poor clinical outcomes for neonates in the long term.

Identifying the Small for Gestational Age Baby at Birth

Newborn babies who are small for their gestational age have a characteristic appearance that typically includes:

• Relatively large head compared to the rest of the body
• Thin appearance with loose, peeling skin
• Decreased muscle mass
• Shrunken and wrinkled facial features
• Loss of subcutaneous fat tissues
• Thin umbilical cord
• Widened cranial sutures and larger-than-normal anterior fontanelle.

(Osuchukwu & Reed 2021)

Catch-up Growth

As Li et al. (2015) say, birthweight remains one of the strongest predictors of perinatal mortality and disability and it’s important to continue to monitor the SGA baby throughout infancy to ensure catch-up growth is achieved.

Most SGA babies achieve catch-up growth by two years of age. This is typically achieved within the first six to nine months of life for approximately 80% of infants born with SGA. Of the remainder, 10 to 15% show slow, attenuated growth, with the remaining 5% to 10% exhibiting a slower catch-up growth pattern between three and five years of age (Cooke, Divall & Radovick 2016).

However, as Osuchukwu and Reed (2021) point out, babies with fetal growth restriction who are small for their gestational age are also at increased risk of other complications such as prematurity, neonatal asphyxia, hypothermia, hypoglycaemia, hypocalcaemia, polycythemia and sepsis. With risks such as these threatening the well-being of both the mother and her baby, it makes sense to invest in the time and resources to ensure mothers enter pregnancy in as healthy a state as possible.

References

• Cooke, D W, Divall, S A & Radovick, S 2016, ‘Chapter 24 - Normal and Aberrant Growth in Children’, in S Melmed, K S Ppoonksy, P R Larsen & H M Kronenberg (ed.), Williams Textbook of Endocrinology, Elsevier, viewed 1 November 2021, https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/small-for-gestational-age
• Cuttler, L, Misra, M & Koontz, M 2016, ‘Chapter 22 - Somatic Growth and Maturation’, in J L Jameson et al., Endocrinology: Adult and Pediatric, Elsevier, viewed 1 November 2021, https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/small-for-gestational-age
• Joseph, F, Hyett, J, Schluter, P, McLennan, A, Gordon, A, Chambers, G, Hilder, L, Choi, S & Vries, B 2020, ‘New Australian Birthweight Centiles’, Medical Journal of Australia, vol. 213, no. 2, pp.79-85, viewed 1 November 2021, https://onlinelibrary.wiley.com/doi/full/10.5694/mja2.50676
• Kildea, S, Gao, Y, Rolfe, M, Boyle, J, Tracy, S & Barclay, L 2017, ‘Risk Factors for Preterm, Low Birthweight and Small for Gestational Age Births Among Aboriginal Women From Remote Communities in Northern Australia’, Women and Birth, vol. 30, no. 5, pp.398-405, viewed 1 November 2021, https://www.sciencedirect.com/science/article/abs/pii/S1871519216302256
• Knight-Agarwal, C, Mellor, D, Georgousopoulos, E, Krause, B & Coghlan, S 2020, ‘Maternal Body Mass Index, Smoking Status and Small for Gestational Age: An Australian Retrospective Cohort Study’, Public Health, vol. 185, pp.381-385, viewed 1 November 2021, https://www.sciencedirect.com/science/article/pii/S0033350620301918
• Li, Z, Umstad, M., Hilder, L, Xu, F & Sullivan, E 2015, ‘Australian National Birthweight Percentiles by Sex and Gestational Age for Twins, 2001–2010’, BMC Pediatrics, vol. 15, no. 1, viewed 1 November 2021, https://link.springer.com/article/10.1186/s12887-015-0464-y
• McRae, D, Janssen, P, Vedam, S, Mayhew, M, Mpofu, D, Teucher, U & Muhajarine, N 2018, ‘Reduced Prevalence of Small-For-Gestational-Age and Preterm Birth for Women of Low Socioeconomic Position: A Population-Based Cohort Study Comparing Antenatal Midwifery and Physician Models of Care’, BMJ Open, vol. 8, no. 10, viewed 1 November 2021, https://bmjopen.bmj.com/content/8/10/e022220
• Osuchukwu, O & Reed, D 2021, ‘Small for Gestational Age’, StatPearls, viewed 1 November 2021, https://www.ncbi.nlm.nih.gov/books/NBK563247/
• University Hospitals Plymouth NHS Trust 2021, Fetal Growth Surveillance in Singleton Pregnancies, University Hospitals Plymouth NHS Trust, viewed 1 November 2021, https://www.plymouthhospitals.nhs.uk/download.cfm?doc=docm93jijm4n6348