Cover Story

Does Doppler U/S improve outcomes in growth-restricted fetuses?

By Alfred Abuhamad, MD

Clearly, that's true for high-risk gestations in general. But while the use of Doppler U/S for managing IUGR pregnancies is creating much excitement, it does have limitations.

Before ultrasound (U/S) arrived on the scene, a diagnosis of intrauterine growth restriction (IUGR) could rarely be made before birth. That has changed dramatically. Today accurate pregnancy dating—precise knowledge of gestational age—is the most important step in prenatal management. It is essential for managing high-risk pregnancies in general and IUGR in particular. U/S plays an integral role in confirming GA, given its ability to pinpoint GA within 3 to 4 days when performed between 14 and 22 weeks' gestation.¹

IUGR is usually broadly defined as an estimated fetal weight below the 10th percentile for GA. Under this definition, however, about 70% of fetuses will be constitutionally small (that is, small for gestational age or SGA), experience no deprivation of oxygen and nutrients, and thus be at no increased risk for perinatal morbidity or mortality.² Therefore, the 5th percentile as a cut-off for diagnosis of true IUGR may be more useful, given the rising numbers of complications and deaths beyond this threshold.³

Of all the U/S-derived biometric parameters, the abdominal circumference (AC) is the most sensitive indicator for IUGR. A diagnosis of IUGR is nearly always on the money (>95% sensitivity) when the AC measurement falls below the 2.5th percentile for GA.^4,5 Therefore, ob/gyns and perinatologists should closely monitor the AC growth profile of fetuses at risk for growth abnormalities and use the appropriate growth curves when estimating fetal weights by U/S. (Curves at high altitudes will underestimate IUGR by about 50%.⁶)

When compared to normal fetuses of the same age, IUGR fetuses have an increased risk of perinatal mortality and morbidity.⁷ During childhood, IUGR fetuses will have higher rates of physical handicap and neurodevelopmental delay, according to long-term follow-up studies.^8,9 But rather than actual birthweight, the best predictor of long-term neurodevelopmental delay appears to be the presence of chronic metabolic acidemia in utero.¹⁰ Timing of the delivery is the most critical step in managing an IUGR pregnancy. Physicians face the serious challenge of finely balancing the risk of prematurity with the risk of long-term neurodevelopmental delay.

Traditionally, physicians managing IUGR pregnancies have relied on cardiotocography for fetal surveillance, during which they look for heart rate variability as a sign of fetal well-being. Heart rate variability involves the complicated interaction between the sympathetic and parasympathetic innervation of the heart. More precisely, it's the end result of the rhythmic, integrated activity of autonomic neurons generated by organized cardiorespiratory reflexes (and modified by arterial neuronal activity).¹¹

The heart rate tracings of growth-restricted fetuses typically show higher baseline heart rates, decreased long- and short-term variability, and delayed maturation of reactivity, researchers have found.^13,14 However, these investigators relied on computer-generated analyses of fetal heart rate tracings. In contrast, the reliability and reproducibility of unaided visual analyses of FHR records appear to be limited.^15,16 Furthermore, by the time overtly abnormal patterns of FHR tracings emerge, they represent late signs of fetal deterioration.^17,18 Therefore, reliance on unaided visual analysis of cardiotocography as the only test of fetal surveillance in IUGR fetuses does not hold up well under scrutiny and won't ensure the best long-term outcome.