Intrauterine growth restriction (IUGR), which is also known as fetal growth restriction, is an important diagnosis to detect in pregnancy due to the high risk of perinatal morbidity and mortality.¹ In fact, a high percentage of stillbirths, both term and preterm, are due to undiagnosed IUGR. Undetected IUGR accounts for a much higher proportion of stillbirth than cases of IUGR detected in the antenatal period.² It is, however, important to understand that not all small fetuses are growth restricted, as they may have in fact met their true growth potential despite their small size. This chapter will attempt to help differentiate constitutionally small fetuses from those that are truly growth restricted and at risk for perinatal morbidity and mortality. It will discuss strategies for the diagnosis and management of IUGR as it pertains to an obstetrics and gynecology (OB/GYN) hospitalist.

IUGR has numerous causes, including maternal, fetal, and placental factors. In many cases, these factors overlap (Table 36-1). The underlying pathophysiology for IUGR is multifactorial and largely dependent on the precipitating etiology. Although most cases are related to placental insufficiency, a careful assessment for fetal conditions, including structural malformations, genetic conditions, and maternal disorders, should be performed. There are relatively few interventions once IUGR has been diagnosed, but careful attention to improving fetal growth through optimizing maternal medical conditions should be considered.

IUGR is a leading cause of perinatal morbidity and mortality, with perinatal mortality rates significantly higher than non-growth-restricted fetuses.^3,4 Studies suggest that 53% of preterm stillbirths and 26% of term stillbirths are growth restricted.⁵ Not surprisingly, the highest risk of morbidity and mortality is found in association with fetuses that are the most severely growth restricted. In addition to stillbirth, growth-restricted fetuses are at increased risk for prematurity, hypoglycemia, hyperbilirubinemia, hypothermia, impaired immune function, interventricular hemorrhage (IVH), necrotizing enterocolitis (NEC), respiratory distress syndrome (RDS), and seizures.^6–8 Also, growth-restricted fetuses are thought to be predisposed to cognitive delay in childhood and metabolic diseases such as cardiovascular disease, obesity, and type 2 diabetes in adulthood.^9,10 Finally, IUGR is also frequently found in association with adverse maternal diagnoses of pregnancy, including the spectrum of hypertensive diseases of pregnancy.

Intrapartum complications are also more frequent in growth-restricted fetuses with reports of higher rates of abnormal fetal heart rate (FHR) tracings and an increased risk for cesarean section (C-section). This may be due to umbilical cord compression in the setting of the oligohydramnios, decreased fetal reserve with placental insufficiency, and increased risk of meconium staining.

Concern for a diagnosis of IUGR is traditionally raised when the estimated fetal weight or abdominal circumference is measured by ultrasound at <10th percentile.¹¹ However, these arbitrary parameters do not differentiate the constitutionally small fetus from the pathologically growth-restricted fetus unable to meet its true growth potential.¹² It has been reported that up to 70% of fetuses with an estimated fetal weight (EFW) <10th percentile will have a normal perinatal outcome.^13–15 Given the diagnostic imprecision, other biometric parameters have also been used (i.e. <5th percentile, <3rd percentile) in an attempt to increase the specificity of the diagnosis. To further complicate the matter, those fetuses that are greater than the 10th percentile, but not meeting their growth potential, will also be missed using these diagnostic criteria. By setting the cutoff too high, there would be poor specificity. However, too low of a specificity would result in poor sensitivity and missed cases of true IUGR.

It is, however, important for the OB/GYN hospitalist to recognize that using the higher cutoff of the 10th percentile will result in a large percentage of normal fetuses (i.e. those that are constitutionally small) being classified as growth-restricted. Furthermore, those that are just constitutionally small are not likely at increased risk for adverse perinatal outcomes. As maternal ethnicity, height, weight, and parity are all contributing variables to fetal size, more recent studies have redefined growth curves that are customized to include parameters characterizing growth based on variables related to the individual fetus. As these customized growth curves are not currently utilized or universally accepted, it is important to include these variables in the overall assessment of the patient.

Nonetheless, from the OB/GYN hospitalist’s perspective, whether it involves making the diagnosis or understanding what parameters were used to identify a known diagnosis, it is paramount to establish the most accurate pregnancy dating during the evaluation. Inaccurate pregnancy dating has the potential for gross misrepresentation of the diagnosis. If the pregnancy is not well dated, other ultrasound parameters may play a role in determining whether a fetus is growth restricted.

Once the best approximation of gestational age is established, there are several ultrasound indices utilized to make the diagnosis of IUGR. In the absence of proper dating information, it may be difficult to distinguish IUGR from incorrect pregnancy dates. In general, a sonographic EFW <10th percentile is most commonly used to identify a fetus at risk for IUGR. However, the abdominal circumference (AC) is probably the single best biometric parameter to establish a concern for IUGR. A normal AC has high specificity and negative predictive value because it is rarely associated with IUGR, even when the composite EFW is <10th percentile.^16,17 Therefore an AC <10th percentile (and particularly <5th percentile) with known dates may be suspicious for evolving IUGR, even when the composite EFW is >10th percentile.

When pregnancy dating is not well established, the transverse cerebellar diameter (TCD) may assist in identifying a fetus that is growth restricted.^18–20 This measurement is not affected by growth restriction and therefore may serve as an independent indicator of gestational age or may be used in comparison to the other biometric markers to help make a diagnosis. This can be used as a unique reference point or internal control because the TCD approximates the weeks of gestation (e.g. 24 mm approximates a 24-week gestation).

In general, it is not advised to redate a pregnancy when gestational age has previously been established. Overall, the most dependable diagnosis of IUGR relies on early pregnancy dating, serial assessments over time (with an optimal interval of 3–4 weeks), and graphic depiction of growth curves and biometric parameters. Fetuses that continue along the same growth trajectory are likely to be constitutionally small, and those falling off their curves are likely to be at higher risk for true IUGR.

Umbilical artery (UA) Doppler is widely accepted as a primary assessment tool for fetuses suspected to be growth restricted.²¹ UA Doppler provides an assessment of the feto-placental interface with respect to resistance to blood perfusion through the umbilical cord. Normally, the low impedance in the umbilical artery permits forward-flow through fetal systole and diastole. When there is increased resistance to blood flow in the umbilical artery, abnormalities in the ratio of flow between systole and diastole develop and progress to absent or reversed end-diastolic flow. Of note, there are gestational age–specific ratios because as a normal pregnancy progresses, there is less resistance at the placental interface. Use of UA Doppler is endorsed by ACOG and other societies due to the associated reduction in perinatal mortality observed.