Acrobat
MS WordPerinatologist Corner - C.E.U/C.M.E. Modules
Fetal Growth Restriction
Sponsored by The Indian Health Service Clinical Support Center
4. Read the material on fetal growth restriction
Diagnosis
Once the diagnosis is established, a work-up for the above-noted conditions may be begun, and can be directed by historical factors and the clinical setting. Nevertheless, a specific etiology is only found in about half the cases. How is the diagnosis best established? The World Health Organization has found that simple sequential measurements of fundal height are perhaps the best screening tool to appraise adequate fetal growth, although their sensitivity is only 50-80%. Lagging fundal growth may be due to a small fetus and/or oligohydramnios, and may be variable as a result of maternal habitus. Ultrasound is currently the mainstay of diagnosing F.G.R.
Critical to F.G.R. diagnosis is accurate early pregnancy dating. A first trimester ultrasound is more accurate than menstrual dating because of the variability in the length of the follicular phase. If the discrepancy between the crown-rump length and the menstrual dates is greater than 7 days, dating preference should be given to sonography. In the second trimester the head circumference is the most accurate parameter, but the composite of multiple parameters will slightly improve accuracy. There are 38 published regression equations to which your ultrasound machine may be programmed, and all are able to give comparably accurate predictions. If the discrepancy is greater than 7-10 days when compared to the L.M.P., give preference to the biometric prediction.
How about the third trimester where most of the F.G.R. cases usually come to attention? Here ultrasound is no longer accurate to within one week. If the patient presents for her first evaluation in the third trimester and you have no other clinical data to use for dating, use the sonographic estimate of gestational age to date the pregnancy. Now the strategy must be to evaluate serial growth. The various biometric parameters cannot very accurately be compared at less than 3-week intervals. Therefore, repeat the scan in 2-4 weeks to see whether or not the fetus is growing along its growth percentile curve, or is "falling off" its curve. Remember that ultrasound-derived estimated fetal weight (E.F.W.) is obtained from the volume of the structures measured; it cannot take into account the variable of tissue density, so in late gestation the E.F.W. may vary by as much as 10-20% from the actual weight obtained at birth. Occasionally, in the situation of a suspected near-term growth-restricted fetus versus a wrongly dated premature fetus, amniocentesis may be permissive of delivery if fetal lung maturity can be documented.
It is unclear whether the distinction between "symmetric" and "asymmetric" F.G.R. has any prognostic significance, but classically, symmetric growth restriction is associated with an early intrinsic insult that is usually not amenable to intervention. Yet is the fetus in question symmetrically growth restricted, or just at a younger gestational age and growing appropriately? It is difficult to discern if you only have one third-trimester scan. On the other hand, asymmetric growth restriction, where the abdominal circumference is smaller than the head circumference (elevated HC/AC ratio), is associated with extrinsic, potentially remediable factors, and its presence should facilitate the diagnosis of actual growth impairment. It is theorized to be the result of utilization of fetal hepatic glycogen stores and fetal intra-abdominal fat stores with a subsequent reduction in liver/abdominal volume.
The other important clue to the ultrasound diagnosis of F.G.R. is the presence of oligohydramnios. Amniotic fluid volume is gestational-age dependent as well, so a nomogram should be consulted at the earlier gestational ages. The familiar criteria for the amniotic fluid index (A.F.I.) of <5 cm for severe oligohydramnios, and < 8cm for "borderline" oligohydramnios, are derived from the study of term fetuses. "Oligo" will usually only develop in the more severely affected cases, however. It is hypothesized to be a result of brain shunting causing relative splanchnic ischemia with resultant decreased renal blood flow and decreased fetal urine production. The A.F.I. is subject to considerable inter- and intra-observer variability, so repeat measurements are usually appropriate before making a clinical decision on this parameter alone.
Altered fetal hemodynamics can also be fairly well assessed by Doppler velocimetry of various fetal vessels, and this less-used ultrasound modality has become very helpful in the contemporary strategy for the diagnosis and management of F.G.R. An increased systolic-to-diastolic ratio (S/D) in the umbilical artery is thought to estimate downstream increased placental resistance reflecting placental insufficiency. Nomograms are available as values are also gestational age-dependent. At term, values >3.0 are thought to be abnormal. By itself, the actual S/D ratio has been less useful than the presence of absent- or reverse-end diastolic flow in the umbilical artery. Both these findings may reflect severe, life-threatening F.G.R., and require some clinical action.
Prior to the development of these advanced disease findings, however, evaluation of the middle cerebral artery (M.C.A.) may allow a better early assessment of the situation. As placental insufficiency and resistance increase, the fetus will shunt blood to its central nervous system, the "brain-sparing effect," and the pulsatility index (P.I.) of the M.C.A. will decrease (a fall in cerebral resistance to allow enhanced flow). The combination of a high umbilical artery P.I. and a low P.I. in the M.C.A. will allow the identification of a compromised fetus prior to end-stage disease. Growth-restricted fetuses with a normal "cerebroplacental ratio" have perinatal outcomes not different from A.G.A. babies. Altered waveforms in various fetal venous structures (umbilical vein, ductus venosus, inferior vena cava) also have prognostic significance. An understanding of how to interpret these studies and knowing when to order them can improve management of these pregnancies.
