All pregnant women should be assessed for small for gestational age (SGA) risk factors in the first trimester of pregnancy but regular measurement of fetal size by ultrasound is only required when major risk factors for impaired fetal growth are identified, says revised guidelines published by the Royal College of Obstetricians and Gynaecologists (RCOG).
A SGA fetus is one that has an estimated weight below a specific threshold. While the majority of term SGA babies have no morbidity and mortality, screening is important as there is an increased risk of stillbirth and neonatal complications in SGA pregnancies compared to appropriately grown babies.
The newly revised Green-top Guidelines recommend that women who have or develop major, or multiple minor, risk factors should be referred for serial ultrasound measurement of fetal size and wellbeing.
Major risk factors for SGA include a previous SGA birth, with a two-fold increased risk of these women having a SGA baby in any subsequent pregnancies, and a prior history of placenta–mediated diseases (including pre-eclampsia and stillbirth).
The guidance recognises that some maternal medical conditions, such as diabetes and chronic hypertension are also associated with a higher risk of a SGA baby. Furthermore, increased maternal age (>40 years) and several maternal exposures including heavy cigarette smoking and cocaine use are major risk factors.
For women presenting with major risk factors, the guidance recommends the use of serial ultrasound assessment of fetal size and fetal wellbeing using umbilical artery Doppler (a form of ultrasound which measures blood flow) from 26-28 weeks of pregnancy.
The guidance also acknowledges that the presence of multiple (3 or more) minor risk factors (such as first pregnancy, low body mass index, long or short intervals between pregnancies) constitutes a risk for a SGA baby and there is a rationale for further screening and refinement of risk in this instance. These women should be referred for uterine artery Doppler at 20-24 weeks, state the guidelines.
In low risk women, who have few or no risk factors, uterine artery Doppler has limited accuracy in predicting a SGA baby and the guidelines recommend it is not used.
In addition to recommending ultrasound surveillance of fetal growth in a selected group of women, the guidance suggests that healthcare professionals also consider interventions for the prevention of SGA babies. Promotion of healthy lifestyle changes, such as smoking cessation, are highly recommended although there is inadequate evidence to recommend dietary modification as a means of preventing SGA births.
Professor Stephen Robson, lead developer of the guidance, said:
“These guidelines incorporate important revisions for healthcare professionals to clarify the risk factors associated with delivering a SGA baby. They should further assist carers identifying pregnancies that need increased fetal surveillance.
“While serial ultrasound screening has moderate predictive value in high-risk women, it has limited accuracy to predict a SGA baby or adverse neonatal outcomes in women without significant risk factors and is therefore not recommended in this population.”
Dr Philip Owen, Chair of the RCOG Green-top Guideline Committee, added:
“These guidelines are a major revision and provide up-to-date information on the risks and screening methods used in SGA pregnancies.
“While they are intended to provide management recommendations, each case should be treated individually and obstetric staff should provide clarification and support as the pregnancy progresses.
“If women are unsure of the risks or management of a SGA baby, they should consult their obstetrician for further information.”