Article Type: Original Article
Abstract:

The second trimester scan is the most important sonographic evaluation during pregnancy. The second trimester ultrasound examination is not only for confirming gestational age but also it provides an ideal opportunity for assessing fetal anatomy and therefore structural normality. In addition, assessment of placental position and morphology, amniotic fluid volume , number of fetuses, evaluation of soft markers for chromosomal defects and the comparative interpretation of various measurements are all important pointers to potential problems. This examination is commonly referred to as a „routine second trimester anomaly scan‟. The optimal time at which to offer the routine anomaly scan is the earliest gestation at which the necessary measurements and a full fetal anatomy survey can be performed and the latest gestation at which an acceptable range of options can be offered to the parents if an abnormality is detected. Although the measurements required to date the pregnancy accurately can be taken after 15 weeks of gestation, and most of the fetal anatomy can be evaluated at 18–20 weeks, the optimal time for examination of fetal heart can be provided at 23-28 weeks. It is recommend that the routine anomaly scan is performed between 20 and 24 weeks but we recommend anomaly scan to be done before 20 weeks (18-20 weeks) in Iran as we have limitation for legal termination if needed. Although it is necessary to examine the entire fetus and other uterine contents in detail, it is not always feasible to do this in the order suggested. It is suggested that the measurements are always carried out early in the examination so that they are not forgotten. It is not reasonable to expect all structural fetal abnormalities amenable to ultrasound detection to be diagnosed at a routine second trimester anomaly scan. Though if the approach is systematic then no major structural abnormality should be missed. However there are several examples from anomalies such as microcephaly which might be missed in second trimester scan if there is no serial examination. In the majority of normal pregnancies, measurement of the biparietal diameter (BPD) and femur length (FL) provide the most accurate assessment of gestational age in the second trimester. It is recommended that measurements of the head circumference (HC), transcerebellar diameter (TCD) and abdominal circumference (AC) are also undertaken. They provide further confirmation of gestational age and aid in the exclusion of growth related abnormalities and spina bifida. In addition, their inclusion encourages a systematic examination of the whole fetus. An ultrasound examination is in the unique position of being both a screening test and a diagnostic test for fetal anomalies. Its clinical value is directly dependent on the skills of the sonographer, first, in obtaining the correct images for evaluation and measurement and, second, in the correct interpretation in each specific and unique clinical situation. Such examinations must only be performed by individuals who have undergone a supervised period of training that enables them to identify and distinguish between the range of normal findings, findings of uncertain significance and abnormalities at varying stages of gestation.