Assessment of gestational age

Most babies are born at term and are healthy (Figure 40.1). However, low birthweight (LBW) babies, that is <2.5 kg at birth, comprised 7% of livebirths in 2014 (ONS 2015). Babies weighing <1.5 kg at birth are classed as very low birthweight and babies <1 kg as extremely low birthweight.

Babies may be LBW because they are born preterm, because they are constitutionally small or because intrauterine growth has not followed the expected trajectory (growth restriction). The baby may at increased risk of perinatal morbidity or mortality. Accurate assessment of gestational age is essential to ensure appropriate care. A useful assessment tool is the Ballard Score (Lissauer et al. 2016).

Preterm babies

A preterm baby is one born alive before 37 weeks’ gestation (Figure 40.2). Worldwide, prematurity is the leading cause of death before the age of 5 years. Subclassifications include very preterm (i.e. 28–32 weeks’ gestation) and extremely preterm: (<28 weeks’ gestation). A baby may be delivered preterm deliberately (elective preterm birth) if it is medically indicated. Examples of such situations are higher-order multiple pregnancies, or maternal or fetal disease where continuation of the pregnancy would be risky for the health of mother or fetus. However, the majority of preterm labours are idiopathic (spontaneous.)

Risk factors for spontaneous preterm birth include:

• Maternal infection
  
• Uterine malformations
  
• Multiple pregnancy
  
• Maternal substance misuse
  
• Domestic abuse.

Small for gestational age babies

The definition of small for gestational age (SGA) is an estimated fetal weight (EFW) or abdominal circumference (AC) below the 10th centile; severe SGA is an EFW or AC below the 3rd centile. The fetus may be constitutionally (‘normally’) small, with growth appropriate for maternal ethnicity or size (RCOG 2013) or the cause may be pathological. SGA is not synonymous with intrauterine growth restriction (IUGR). Growth restriction implies the presence of pathology that is interfering with the fetal growth potential (Figure 40.3). Pathologies may be non-placenta mediated such as fetal anomalies or infection, or placenta-mediated such as maternal disease that may affect transplacental transfer of nutrients.

Risk factors for IUGR include:

• Previous SGA baby, stillbirth or pre-eclampsia
  
• Maternal substance misuse
  
• TORCH infections (toxoplasmosis, other – syphilis, parvovirus, varicella-zoster, rubella, cytomegalovirus, herpes)
  
• Maternal disease such as pre-existing diabetes, renal disease or chronic hypertension
  
• Repeated antepartum haemorrhage
  
• Chromosomal anomalies.