SP Chauhan, SF Charania, RA McLaren, et al (Med College of Georgia, Atlanta; Univ Illinois, Peoria; Univ of Mississippi, Jackson)
Ultrasonographic Estimate of Birth Weight at 24 to 36 Weeks: A Multicenter Study.
Am J Obstet Gynecol 179: 909-916, 1998.
Background: A method for reliably estimating fetal weight in preterm labor with eventual preterm delivery would be useful. The accuracies of U.S. estimates of birth weights among infants born between 24 and 34 weeks’ gestation at 3 centers were determined.
Data on 171 fetuses at each center were analyzed.
Prediction limit calculation showed that fetal weight estimates of more than 1,600g, more than 1,900g, and more than 1800g at the 3 centres, respectively, were required to predict actual weights of more than 1500g with a 70% accuracy.
Christine Comstock, MD., Assistant Professor, Wayne State University comments: Accurate estimates of fetal weight can be very useful in clinical decision making in the preterm labor patient. There are a number of methods of estimating weight by ultrasonography. All involve ultrasonographic measurement of at least 2 of the following parameters: biparietal diameter, head circumference, femur length, and abdominal circumference.
In general, an estimate should be within ±10% in 90% of estimates, although in macrosomic fetuses most formulas perform less well. The abdominal circumference, which is used in almost all formulas, is measured by tracing the abdomen at the level of curvature of the hepatic vein in a plane exactly perpendicular to the spine and includes the actual skin line. If the fetus is breathing or if it is lying against the uterine wall or if there is little amniotic fluid, the skin line is not stable or cannot be seen well.
The 24 to 34 weeks window is the one in which most centers have been successful in predicting weight in the absence of ruptured membranes. Amniotic fluid is most plentiful during those weeks, providing a clear outline of the abdomen, and macrosomia is rarely present. This is the window when a carefully obtained weight estimate should be expected to be within ± 10% in most cases.
L Verma, F Macdonald. P Leedham, et al (Birmingham Heartlands Hosp, England)
Rapid and Simple Prenatal DNA Diagnosis of Down’s Syndrome.
Lancet 352: 9-12, 1998.
Since the 1970s, cultured amniotic fluid cells obtained by amniocentesis at around 16 weeks of gestation have been used for the prenatal diagnosis of down’s syndrome by Karyotyping. This approach requires 10 mL of fluid, and it takes an average of 15 days for the results to be known. A new approach involves polymerase chain reaction (PCR) amplification of small-tandem-repeat markers located on chromosome 21 and analysis with fluorescence based methods. With this technique, results are available on the same day and only a small amount of fluid is required. The use of this approach for the diagnosis of trisomy 21 was investigated in a series of more than 2000 samples of amniotic fluid
Two DNA markers gave an informative and correct result in 2,083 of 2,139 samples (97.4%), in which 30 fetuses were identified as having trisomy 21 Down’s syndrome and 2,053 fetuses were identified as normal. In 32 of 41 other clear samples, an extra marker was informative. With these 3 markers, a total of 99.6% informative results were achieved. No false negative or false positive results were seen.
Authors conclude -For improved prenatal diagnosis of Down’s syndrome, the PCR-based DNA diagnostic test has great potential. The advantage is that results may be available within a day.
The PCR has revolutionized the practice of molecular genetics. Similar techniques could be employed simultaneously for the prenatal diagnosis of other common aneuploidies such as trisomy 13 and trisomy 18. PCR based prenatal diagnosis of trisomy 21 (as an adjunct to conventional cytogenetic analysis) is cost-effective and that the tremendous time saving. Rapid prenatal diagnosis of trisomy 21 through fluorescence in situ hybridization (FISH) is already a relatively well-established procedure in the United States.
MJ Hussey, ES Levy, et al (Rush-Presbyterain-St.Luke’s Med Ctr, Chicago)
Evaluating Rapid Diagnostic Tests of Intra-amniotic Infection: Gram Stain, Amniotic Fluid Glucose Level, and Amniotic Fluid to Serum Glucose Level Ratio.
Am J Obstet Gynecol 179:650-656, 1998.
After studying one hundred twenty-seven patients in preterm labor and 26 with preterm premature rupture of the membranes, the authors came to the conclusion that the diagnostic values of amniotic fluid glucose and the ratio of amniotic fluid to serum glucose were equivalent. However, Gram stain is superior to both. Combining Gram stain with amniotic fluid glucose level is superior to any individual test.
Ymd Lo, NM Hjelm, C Fidler, et al (Univ of Hong Kong, China; John Radcliffe Hosp, Oxford, England)
Prenatal Diagnosis of Fetal RhD Status by Molecular Analysis of Maternal Plasma.
N Engl J Med. 339: 1734-1738, 1998.
Rhesus (Rh) isoimmunization still occurs, despite the widespread use of Rh immune globulin prophylaxis in RhD negative pregnant women. It would be useful to determine the RhD status of these infants because no further testing or therapeutic procedures would be necessary if the infants were RhD-negative. The feasibility of fetal RhD genotyping with the use of fetal DNA extracted from plasma samples from RhD negative pregnant women was assessed.
Method: The study included 57 RhD negative pregnant women and their singleton fetuses: 12 were in their first trimester; 30 were in their second trimester; and 15 were in their third trimester. An analysis was conducted for the RhD gene with a PCR test sensitive enough to detect the RhD gene in a single cell from DNA extracted from maternal plasma. Serologic analysis of cord blood or PCR analysis of amniotic fluid was used to determine fetal RhD status.
Results : The results of RhD PCR analysis of maternal plasma DNA were completely concordant with the results of serologic analysis in the samples obtained from women in their second or third trimester of pregnancy. Two of the maternal plasma samples collected in the first trimester contained no RhD DNA, but the fetuses were RhD-positive. The other 10 samples were found to be concordant; 7 were RhD-positive and 3 were RhD negative.
Conclusion: Noninvasive fetal RhD genotyping can be performed rapidly and reliably with the use of maternal plasma at the beginning of the second trimester of pregnancy. Not only is the method is reliable but also rapid.
TD Shipp, BR Benacerraf (Massachusetts General Hospital, Boston; Brigham and Women’s Hospital, Boston; Harvard Med School, Boston)
The Significance of Prenatally Identified Isolated Clubfoot: Is Amniocentesis Indicated?
Am J Obstet Gynecol 178:600-602, 1998.
Most fetuses with clubfoot have other malformations, often associated with karyotypic abnormalities.
Patients: The 9-year review included 87 fetuses with isolated clubfoot identified on prenatal Ultrasound. Follow-up information was available on 68 fetuses. At birth, 38 fetuses were correctly identified as having bilateral clubfoot and 15 as having unilateral clubfoot. Eight fetuses with a ultrasound diagnosis of clubfoot were found to be normal at birth. In utero karyotyping was performed on 34 fetuses, 4 of which had abnormal karyotypes. The abnormal karyotypes identified were 47 XXY, 47 XXX trisomy 18, and trisomy 21.
The authors conclude that a 6% incidence of karyotypic abnormalities among fetuses with a ultrasound finding of isolated clubfoot was found. These fetuses should be karyotyped even if clubfoot is the only apparent congenital anomaly. Ultrasound used prenatally for the detection of clubfoot has a false-positive rate of 12%; the false-negative rate is unknown.
Budorick et al report on the value of 3-dimensional ultrasound of the fetal distal lower extremity, normal and abnormal. Three-dimensional ultrasound improves one’s ability to evaluate the lower limb.
AG Hadley, A Wilkes, et al (Internatl Blood Group Reference Lab, Bristol, England; The Bristol Blood Centre, England; St. Michael’s Hosp, Bristol, England)
The Ability of the Chemiluminescence Test to Predict Clinical Outcome and the Necessity for Amniocenteses in Pregnancies at Risk of Haemolytic Disease of the Newborn.
Br. J Obstet Gynaecol 105: 231-234, 1998.
The results of the study show that chemiluminescence testing was better correlated with the clinical outcome of hemolytic disease than was AutoAnalyzer testing. Chemiluminescence correctly identified 94% of unaffected fetuses, compared with 73.5% with the AutoAnalyzer. Prediction of severely affected cases were 97% vs.85% respectively. Neither test correctly distinguished unaffected from mildly affected cases.
Comments: The first step is to establish antibody levels in maternal serum to determine if amniocentesis or percutaneous umbilical cord sampling is indicated. Traditionally, anti-D levels have been measured by AutoAnalyzer, but cellular assays such as the chemiluminescence test (CLT) measure the biological activity of the antibodies; thus, they should be more reliable.