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Prenatal diagnosis should be instituted accurately and early enough to achieve an excellent level of care of the obstetric patient. Ultrasonography, with its increasingly better resolution and image quality, is making it easier for health care providers to make appropriate management decisions. Assessing whether a structure is normal or abnormal may not always be feasible, but if doubt exists regarding its normalcy, it must be carefully and diligently pursued. In addition, several measurements, such as the biparietal diameter (BPD) and the head circumference (HC), can be used to determine the gestational age of a fetus, especially during the first half of pregnancy. Correct estimation of gestational age is of paramount importance because adequate management of both low- and high-risk obstetric populations relies heavily on knowing the precise gestational age.

The tables in this chapter have been compiled from the literature for the sole purpose of serving as an easy reference against which measurements can be compared. An attempt has been made to include as many tables of different parameters as possible. These tables, assembled in a single chapter, will allow the sonographer or sonologist to more easily make the differentiation between normal and abnormal measurements, without having to search different textbooks and articles for a particular measurement.

This chapter is divided into four main sections. The first, crown-rump length (CRL), although not a brain or head measurement, is included to provide a means of predicting embryonic or fetal age. This parameter is of obvious importance and eliminates the need to turn to another source.

The second section, dealing with head measurements, can be used not only to date the pregnancy but also to aid in the diagnosis of microcephaly and alterations in fetal head shape. This section also includes measurements of orbital diameters, which can be of help in the diagnosis of eye pathology.

The third section provides a variety of tables concerning the different portions of the fetal ventricular system, mainly for the purpose of making early diagnosis of ventriculomegaly possible. Congenital hydrocephaly is one of the most frequently described anomalies, with an incidence of 0.3 to 1.5 per 1000 births. The importance of in utero detection of this anomaly cannot be overemphasized. This section also includes tables that we have generated using the transvaginal-transfontanelle approach to the fetal brain using 5 to 7.5 MHz transvaginal probes. These tables enhance and complement the widely accepted transabdominally generated tables, thereby advancing the field of fetal neurosonography and giving new meaning to the term early diagnosis.

The fourth and final section includes measurements of other intracranial structures, such as the thalami, basal nuclei, cerebellum, and cerebellomedullary cistern. These can assist the sonographer or sonologist in the diagnosis of pathologies such as Dandy-Walker and Arnold-Chiari malformations.

This chapter attempts to provide the reader with a unique reference guide to fetal brain measurements.

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