First and second trimester sonography: an American perspective.

Abstract

In the United States, first-and second-trimester ultrasonography is most commonly used for gestational dating, detection of fetal aneuploidy, identification of early fetal intrauterine growth restriction (IUGR), and assessment for cervical incompetence. Crown-rump length (CRL) between 7 and 12 weeks is the most accurate parameter for first-trimester dating. In the second trimester, the biparietal diameter, head circumference, transverse cerebellar diameter (TCD), abdominal circumference, femur length, and other long bones, such as tibia and humerus, are useful. The TCD appears to be particularly useful because of its relative sparing in IUGR. Ultrasound can aid in the detection of fetal aneuploidy by identifying structural anomalies or abnormal fetal biometry in the first and second trimester. Numerous structural abnormalities are suggestive of aneuploidy. Cystic hygroma and nuchal translucency appear to be most significant first-trimester markers for fetal aneuploidy. Second-trimester estimated fetal weight (FFW) curves have been developed and are useful in the early detection of IUGR. Second-trimester FFW curves are useful for the detection of trisomy 18 (sensitivity 60%) but not for trisomy 21 (sensitivity 8-12%). Fetal biometry of long bones is also useful in identifying fetuses at risk for aneuploidy. Identification of a second-trimester fetus with either humerus or femur shorter than expected places the fetus at risk for aneuploidy. The sensitivity of short long bone in detection of fetal aneuploidy is approximately 30%, with false positive rates < 5%. Nuchal fold thickness > 6 mm in the second trimester is also used for identifying aneuploid fetuses. The overall sensitivity for the detection of Down's syndrome in fetuses with increased nuchal fold thickness is approximately 34% and the false positive rate is 1.5%. We have developed a model by using an ultrasound examination to adjust the mid-trimester risk for trisomy 21 by combining maternal age or triple screen risk assessment (unconjugated estriol, alpha fetoprotein, and human chorionic gonadotropin) and ultrasound. Using this model, the risk for Down's syndrome is found to be increased with identification of abnormal biometry or anomalies, or decreased with a normal genetic ultrasound examination. Another important application is the use of abdominal and transvaginal ultrasound in the second trimester in pregnancies at risk for premature cervical dilatation, premature delivery, and cervical incompetence. We have found transfundal pressure to be useful in the diagnosis of otherwise clinically inapparent premature cervical dilatation and cervical incompetence.