Fetal Growth and Surveillance

Introduction

The focus of this chapter is on fetuses who appear to be small for their gestational age. These babies may simply be small—in other words, they are normal unborn babies who just happen to be at the lower end of a normal range (constitutionally or genetically small) or they may be small for a pathological reason. These latter fetuses are referred to as being affected by fetal growth restriction (FGR, previously called ‘intrauterine growth restriction’). As a group, small-for-gestational-age (SGA) fetuses are at increased risk of perinatal mortality but most adverse outcomes are within the group affected by FGR.

The key issues are how to screen a low-risk population in order to identify these small fetuses and, once identified, how best to identify those that are risk of developing problems in utero or in labour.

Accuracy of Dating

The reliable diagnosis of SGA and FGR requires knowledge of gestational age. The estimated date of delivery is calculated as 40 weeks after the date of the start of the last menstrual period (LMP), provided that the cycle length is 28 days. A correction may be made for those with regular longer or shorter cycles. For example, if the cycle is 35 days long, then 7 days should be added to the date of the LMP. However, menstrual dating has inherent potential inaccuracies: the dates may be inaccurately recalled, the cycle may be irregular, and bleeding in early pregnancy may be mistaken for a period. Abdominal palpation is an inaccurate way of establishing gestational age, as is the date that fetal movements were first noted. Gestational age is most accurately determined by an ultrasound scan undertaken before 20 weeks’ gestation, as it is reasonable to assume that all fetuses of a given gestational age are of a similar size up until this point. The natural variation in size after this stage makes pregnancy dating less accurate. The most reliable fetal measurements for dating are the crown rump length between the 10th and 14th weeks and the head circumference between the 15th and 20th weeks.

The rest of this chapter will assume that gestational age is reliably established.

Small for Gestational Age

SGA describes the fetus or baby whose estimated fetal weight or birth weight is below the 10th centile. As the centile reduces, for example, to the fifth or third, the risk of adverse outcome increases.

Fetal Growth Restriction

The term ‘FGR’ indicates ‘a fetus which fails to reach its genetic growth potential’. FGR presents as a fetus whose growth on serial ultrasound scanning falls below a certain threshold. This threshold is poorly defined and is often implied as the ‘crossing of centiles’ on a chart of fetal biometry (see later discussion).

Babies with FGR appear thin, as quantified by the ponderal index (the ratio of body weight to length) and their skin-fold thickness, a measure of subcutaneous fat, is reduced. There is clearly an overlap in the categorisation of small and/or growth-restricted fetuses; a proportion of SGA fetuses will be growth restricted, but the majority will be constitutionally small, that is, genetically determined to be small. Some growth-restricted fetuses will not be SGA, that is, their growth is failing but they do not have a size below the 10th centile.

Aetiology

Fetal growth is determined by the baby’s intrinsic genetic potential, which is then modified by various fetal, maternal and placental factors ( Box 27.1 ).