Prematurity and fetal growth restriction

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Abstract

Assessment of the growth status of the fetus and neonate is an essential component of perinatal care. It requires a distinction to be made between physiological and pathological factors, and the prediction of the optimal growth that a baby can achieve in a normal, uncomplicated pregnancy. Such an individually customised standard can now be easily calculated by computer: it needs to be accurately dated, individually adjusted for physiological characteristics, exclude pathological factors such as smoking, and be based on a fetal weight trajectory derived from normal term pregnancies.

Application of a customised standard to calculate the growth status of preterm babies gives us freshly insights into the causes of prematurity. Fetal growth restriction is seen as a strongly associated factor, which is often present before the onset of spontaneous preterm labour. This raises the question whether, in many instances, the initiation of parturition should be seen as a fetal adaptive response aimed at escaping an unfavourable intrauterine environment. These concepts have implications for the understanding of the pathophysiology of preterm labour, as well as its clinical management.

Introduction

Prematurity is the leading cause of neonatal morbidity and mortality. There is a large aetiological heterogeneity [1], with many factors known prior to pregnancy that are associated with subsequent preterm birth. These include maternal characteristics such as low maternal age, nulliparity, high parity, low maternal weight, obesity, ethnic origin, social class and cigarette smoking [2], [3], [4].

Social class (including indicators of socio-economic status, education, marital status and low income) is linked to preterm birth [2], [7], [8], [9], [10]. Another factor is ethnic origin; each of the main ethnic groups represented in the UK has an increased likelihood of preterm delivery [5], [12]. It is uncertain as to what degree this difference is due to physiological variation. There is interaction with other confounders, in particular social class, and different interactions are found in different ethnic groups [12]. In Afro-Caribbean women, half of the excess in prematurity rates can be explained by deprivation and marital status, while in Asians from the Indian subcontinent, the increased risk of preterm delivery may not be related to deprivation [12].

The link between pre-pregnancy obesity and preterm delivery is well established [4]. Obese women also have a higher incidence of pre-eclampsia [13], [14], [15]. The effect of smoking on preterm birth is dose related [6], [7]. However, the association is somewhat complex as smoking is also known to be protective of pre-eclampsia in primiparae [7]. Previous obstetric history is also relevant: miscarriage [16] and preterm birth [17], [18] are strongly linked. In fact, the risk is higher, the earlier the previous preterm birth has occurred [19]. Previous preterm birth associated with pre-eclampsia is a risk factor for subsequent indicated preterm delivery associated with pre-eclampsia [20], [21].

Section snippets

Menstrual history versus ultrasound biometry

The proportion of births which fall into the ‘preterm’ category depends on the method of dating used. Over the last two decades, ultrasound scanning has gradually been introduced into routine practice. Improved pregnancy dating by fetal age (as assessed by ultrasound biometry) in contrast to menstrual age and its associated error gives us new perspectives at both extremes of gestation [22]. Many apparently post-term pregnancies are in fact misdated and not post-term by scan dates, as conception

Assessment of the growth status of the baby

A number of investigators have reported a link between small-for-gestational-age (SGA) babies and preterm labour [31], [32], [33], [34], [35], [36]. The actual contribution of fetal growth restriction however has been unknown, as SGA is not equal to FGR. The contributing effect needs to be studied with a growth standard that is based on the ‘normal’, i.e. the optimal growth of each baby. Effective differentiation between optimal and sub-optimal weight requires such a standard to be:

  • 1.

    Accurately

Evidence for customised assessment

Application of an individually adjustable standard for fetal growth allows better distinction between normal and abnormal smallness. This applies both in the antenatal assessment of estimated fetal weight and in the postnatal assessment of birthweight.

Ultrasound based fetal weight curves reproduce the differences due to physiological and constitutional characteristics, in low risk [46] as well as high risk [47] populations. Customised limits reduce false positive ‘IUGR’ in a normal population

Growth restriction and spontaneous preterm labour

Applying an individually optimised standard helps with our understanding of the pathophysiology of preterm birth. Customised centiles have demonstrated the link between prematurity and growth restriction in preterm babies born after spontaneous onset of labour in case-controlled studies in Europe [9] and in the USA [54]. After adjustment for confounders, spontaneous preterm birth was also significantly associated with increased risk of the fetus's not having reached its growth potential [55].

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