Issn Print: 0029-7828

Publication Date: 1997/10/01


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Comparison Between the Single Deepest Pocket and Amniotic Fluid Index in Predicting Fetal Distress in Small-for-Gestational Age Fetuses

Tsunetake Miyamura; Hideaki Masuzaki; Masashi Miyamoto; Tadayuki Ishimaru

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Excerpt

The small-for-gestational age (SGA) fetus is at increased risk of severe distress during labor-especially if chronic uteroplacental insufficiency has led to impaired fetal growth. The constitutionally small fetus, in contrast, may not be at risk of distress, making it important to distinguish between these two states. It remains unclear whether a reduced amniotic fluid volume is typical of high-risk fetuses. If so, it might prove a useful means of predicting distress in SGA fetuses. This study utilized two methods of estimating amniotic fluid volume in 69 singleton pregnancies where a structurally normal infant was delivered within a week of the last estimate, weighing less than the 50 percentile. Both the single deepest pocket (SDP), or cord-free pool, and the amniotic fluid index (AFI) were estimated by uterine ultrasonography. Measurements made in the four uterine quadrants were summed to determine the AFI.
The two methods of estimating amniotic fluid volume correlated very significantly from 24 to 41 weeks' gestation. Fetal distress, detected by fetal heart rate monitoring, developed in 16 cases (23 percent). Using a cutoff value of 3.0 cm, the SDP was the most accurate means of predicting distress. Fetal distress was noted in 48 percent of 21 cases with an AFI less than 8 cm. In 30 cases followed over 2 weeks, the SDP but not the AFI decreased significantly before delivery in cases of fetal distress. Emergency cesarean delivery was carried out in 19 percent of 21 cases with an AFI less than 8 cm, and in 54 percent of 13 cases with a SDP less than 3 cm. Low Apgar scores were recorded in two infants in each group. A SDP less than 3 cm is the most reliable evidence that significant oligohydramnios is present, which may culminate in distress of an SGA fetus.
(Amniotic fluid volume has been shown to be an indicator of fetal well-being. Of course, the rationale is that in response to uteroplacental insufficiency and chronic hypoxia, compensatory redistribution of blood flow occurs in the fetus, resulting in a decrease in perfusion of viscera (including the kidneys) in favor of cerebral, cardiac, and adrenal perfusion. The decrease in renal blood flow results in a decrease in fetal urine production and, therefore, a decrease in AF volume.
The problem is how to best measure AF volume and assure reasonable predictive value for the test. The present study conducted at the Nagasaki University School of Medicine found that in fetuses with evidence of growth delay (i.e., below the 50th percentile), the SDP method using a cutoff value of 3.0 cm was more predictive of fetal distress in labor than was the AFI of <8.
Unfortunately, problems with the methods used and the interpretation of the data prevent this study confirming either the SDP or the AFI as a useful predictor of fetal outcome. The ultrasonographer who measured the AFI also measured the SDP, introducing substantial bias for comparing the methods. Also, the clinicians caring for the patients were not blinded to the AF measurements. Furthermore, the end point was fetal distress manifested by fetal heart rate abnormalities, but not confirmed by other objective measures of fetal hypoxia such as cord blood gas values or the need for infant resuscitation. Finally, the number of patients with SDP <3 cm and with AFI <8 cm, two and nine patients, respectively, is so small as to cast doubts on the statistical validity of conclusions drawn from such data. At best the positive predictive value of oligohydramnios (SDP <3 cm or AFI <8 cm) was 50 to 60 percent; whereas the negative predictive value was 85 to 90 percent.
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