The epidemiological factors described above might be used to identify fetuses which are likely to have growth abnormalities allowing an increased level of surveillance. However, although many statistical associations are described, few of these are particularly strong. Therefore, although a study may show that a woman with a body mass index of 17 has an increased risk of delivering an SGA infant, the majority of these women would deliver an AGA infant. Most adverse pregnancy outcomes occur to women with no identified risk factors. These statements can be expressed in terms of screening: maternal history has low sensitivity and low positive predictive value in detecting fetal growth disorder. Biochemical prediction of IUGR has been investigated primarily using analytes measured in the first or second trimesters in the context of Down's screening programmes. While low first trimester PAPP-A levels are associated with low birthweight, studies of first and second trimester AFP, hCG and inhibin-A have shown a less consistent picture. None of these biochemical analytes has sufficient predictive value to be useful in a clinical context.
Uterine artery Doppler allows indirect assessment of downstream resistance in the arteries, arterioles and capillaries of the maternal side of the placenta. It is a quick, y
Fig. 19.4 Likelihood ratio for severe adverse outcome (vertical axis) relating to mean pulsatility index (horizontal axis). Smokers are represented by a thick black line (to left), non-smokers by a thin line. From Lees et al. (2001) Obstet Gynecol 98(3), 369-73.
simple and non-invasive technique which involves the placement, using colour Doppler ultrasound, of a sample gate over the uterine artery just distal to its origin from the internal iliac artery. Pulsed wave (PW) Doppler is then applied, and a flow velocity waveform is obtained from which resistance indices such as resistance index, pulsatil-ity index (PI) and A/B or S/D ratios can be derived. Low-resistance waveforms indicate good trophoblast invasion to the spiral arterioles, whereas high-resistance waveforms (characterized by low levels of end-diastolic flow and notches) indicate abnormal placentation (Fig. 19.3). The higher the uterine artery PI, the higher the risk of severe adverse outcome due to abnormal placenta-tion (Fig. 19.4). Large studies with good reproducibility reported since the late 1990s have suggested its potential utility as a screening tool from as early as 12 weeks to, optimally, 24 weeks in predicting both pre-eclampsia and fetal growth restriction.
Although the sensitivity of uterine Doppler is poor for growth restriction at any gestation, it becomes better for the more severe and early onset forms. For example, its sensitivity in predicting IUGR<10th centile requiring delivery before 34 weeks is around 80%, for a 5% screen positive rate in an unselected population. It is, however, less reliable at predicting IUGR in twins and if performed earlier in gestation. It has not become a feature of routine antenatal care as controversy still exists over its utility in screening low risk populations, although concerns over reproducibility have now been largely overcome.
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