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    Using ultrasound to recognize fetal anomalies: Part 1

    First- and second-trimester ultrasounds are key prenatal tools for revealing structural anomalies that may point to genetic conditions.

     

    Limbs

    At a minimum, all limbs should be documented, including proximal long bones (humerus and femur) and distal long bones (radius/ulna and tibia/fibula). Severe skeletal dysplasias, such as thanatophoric dysplasia, can have features such as small limbs and narrow chest even in the first trimester. Hands and feet can also be imaged, although abnormalities of the digits cannot always be identified. In some cases, polydactyly (Figure 8) may be suspected.

    Risk factors for structural abnormalities

    Abnormal NT is associated with a higher rate of structural abnormalities in fetuses with normal and abnormal karyotypes. Abnormal NT at 11-14 weeks is an indication for genetic counseling, and early second trimester ultrasound at 14-16 weeks should be considered. Abnormal NT is also an indication for fetal echocardiography in the second trimester.

    In some cases, first-trimester ultrasound findings may be suspicious, but not diagnostic, for abnormalities of certain structures. In these cases, early second-trimester ultrasound can lead to prenatal diagnosis prior to the routine 18- to 20-week scan. In our experience, early second-trimester ultrasound in such cases contributed to lower gestational age at abortion in women undergoing ultrasound at 11 to 14 weeks.4

    Second trimester

    At 18 to 20 weeks, detailed ultrasound can evaluate most anatomic structures. Earlier second-trimester anatomic evaluation at < 18 weeks should be limited to high-risk patients and/or specialized centers.

    Fetal brain/skull

    Imaging in the transverse plane at the level where the biparietal diameter (BPD) and head circumference (HC) are measured allows identification of the cavum septum pellucidum, third ventricle, and thalami. An oblique/transverse view will image structures in the posterior fossa. Finally, a transverse view superior to the midbrain can identify the lateral ventricles. Realtime scanning should evaluate the integrity of the calvarium. Anomalies that should not be missed in the first trimester should not be missed in the second trimester, such as holoprosencephaly (Figure 9), cephalocele (Figure 10), and anencephaly (Figure 11).

    Enlarged ventricles, or ventriculomegaly can indicate several conditions (Figure 12). As an isolated finding, ventriculomegaly can represent obstruction of the flow of cerebrospinal fluid (CSF), and can indicate hydrocephalus. Abnormal cortical development can result in ventricular enlargement in the absence of obstruction.

    Ventriculomegaly is also present in the Dandy-Walker malformation (Figure 13), a malformation characterized by absence of the middle portion of the cerebellum, known as the vermis, with cystic dilation of the fourth ventricle visible in the posterior fossa. Ventriculomegaly is also part of the Arnold-Chiari Type II malformation, which is present in most cases of spina bifida. The spinal defect with herniated tissue, or meningomyelocele, has a negative pressure effect on the brain, with herniation of the hindbrain into the spinal canal. This causes the cerebellum to be elongated and distorted into a “banana” shape” (Figure 14), the frontal bones of the calvarium to collapse, causing a “lemon shaped” skull (Figure 15), and obstruction of CSF flow leading to ventriculomegaly. Evaluation of the spine in sagittal and transverse views is necessary to confirm the presence and level of the spinal defect.

    Related: Second trimester ultrasounds of fetal anomalies in the head and chest

    Ventriculomegaly, whether isolated or with associated structural abnormalities, is associated with genetic abnormalities. Mild ventriculomegaly is associated with Down syndrome. Hydrocephalus in a male fetus with no associated finding could reflect a mutation in the L1CAM gene associated with X-lined hydrocephalus. Dandy-Walker Malformation is associated with chromosomal abnormalities, as well as single gene disorders. While open neural tube defects are usually multifactorial in origin, they may be a feature of Trisomy 18. Genetic counseling is indicated when ventriculomegaly is identified.

    NEXT: More second trimester abnormalities

    Stephen T Chasen, MD
    Dr Chasen is Professor of Clinical Obstetrics and Gynecology in the Department of Obstetrics and Gynecology at Weill-Cornell Medical ...
    Daniel W Skupski, MD
    Dr Skupski is Professor of Clinical Obstetrics and Gynecology in the Department of Obstetrics and Gynecology at Weill-Cornell Medical ...

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