Central Nervous System Anomalies Detectable in the First Trimester

Prenatal diagnosis of central nervous system (CNS) congenital anomalies is possible in the second and third trimester. Earlier diagnosis is possible for some of these as the complex acranial/exencephaly/anencephaly and alobar holoprosencephaly (HPE). The knowledge of normal fetal brain development and its ultrasound images at the different weeks’ gestation, the expertise of operators, and high-resolution equipment are essential to obtain good results. The diagnosis in the first trimester of some major anomalies incompatible with extrauterine life or associated with severe handicap is useful to inform the couple, to perform additional examinations, and provide them with the option of earlier and safer pregnancy termination. For some other conditions, like agenesis of corpus callosum and hypoplasia or absence of cerebellar vermis, the diagnosis is possible only in the second trimester, so beware not to falsely reassure or scare expecting parents. In conclusion, first trimester ultrasound could be useful for early detection of some CNS anomalies, but caution could be used for others, particularly in counseling couples about prognostic significant of ultrasound findings.