Prenatal diagnosis should be instituted accurately and early enough to achieve an excellent level of care of the obstetric patient. Ultrasonography, with its excellent resolution and image quality, makes it easier for health care providers to reach appropriate management decisions. Assessing whether the size of 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 also 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 prevents 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/macrocephaly and alterations in fetal head shape. In addition, this section also includes measurements of orbital diameters, which can be of help in the diagnosis of eye pathology (see also Chapter 19).
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–transfontanellar approach to the fetal brain using 5- to 12-MHz transvaginal probes. These tables enhance and complement the widely accepted transabdominally generated ones, therefore advancing the field of fetal neurosonography and giving new meaning to the term “early” diagnosis.
The fourth and final section includes measurements of other supratentorial structures, such as the corpus callosum thalami and basal nuclei as well as infratentorial structures.
In conclusion, this chapter provides the reader with a unique reference guide to fetal brain measurements.