Need for developing case definitions and guidelines for data collection, analysis, and presentation for fetal growth restriction as an adverse event following immunization:
Fetuses that fail to meet their growth potential in utero are at risk for adverse antenatal and postnatal events such as stillbirth, preterm birth, and adverse neonatal and long-term health outcomes [1], [2], [3], [4], [5]. Therefore, antenatal recognition and monitoring of fetal growth restriction (FGR) is an important component of prenatal care [6], [7], [8]. Despite the clinical and public health importance of this problem there is no universally accepted definition of FGR [9], [10]. Furthermore, terminology such as intrauterine growth restriction (IUGR) or small for gestational age (SGA) are used interchangeably and without specificity to describe this clinical entity. In its simplest form, FGR is defined as a sonographic estimation of fetal weight below the tenth percentile for a given gestational age [11], [12], [13], [14]. Though this definition is simple to understand and translating into practice, it is an inadequate definition for FGR.
FGR can be a consequence of maternal, fetal, or placental factors. Diagnosing all fetuses with an estimated fetal weight (EFW) below the tenth percentile with FGR fails to account for the individual growth potential of each fetus. Constitutionally small fetuses who might be expected to have a lower birthweight based on parental characteristics may be misdiagnosed as pathologically small [15]. Conversely, fetuses destined for a higher birthweight may fail to reach their growth potential due to a pathologic process yet never fall below a threshold based on fetal or birth weight below a specific centile (e.g. 10th) [16]. An ideal definition of FGR would detect those fetuses with a pathologic failure to meet their growth potential subsequently at risk of adverse outcomes.
Numerous studies have attempted to improve the sensitivity and specificity of the definition through adjunct testing and optimization of growth curves used to define the tenth percentile diagnostic cutoff. The sentinel investigations into FGR used measurements of the fetal head, abdomen, and femur to develop growth curves within small homogenous patient populations [17]. Though these measurements yielded reliable estimations of fetal weight, the growth curves lacked generalizability, particularly in an international context [18]. Contemporary studies on FGR have advocated individualized growth curves accounting for maternal and fetal characteristics such as ethnicity and gender to solve this dilemma [19], [20], [21]. However, large-scale international prospective studies of healthy pregnancies show little difference in growth curves between populations [22]. Additional studies investigating the utility of adjunct studies such as amniotic fluid assessment and use of Doppler attempt to further clarify the definition of FGR [23], [24].
