At 32 weeks, the fetus has attained a crown-rump length approximating 28 cm and a weight of about 1800 g. he skin surface is still red and wrinkled. In contrast, by 36 weeks, the fetal crown-rump length averages about 32 cm, and the weight approximates 2800 g (Duryea, 2014). Because of subcutaneous fat deposition, the body has become more rotund, and the previous wrinkled facies is now fuller. Normal fetuses have nearly 100-percent survival rate.
This is considered term, and the fetus is now fully developed. The average crown-rump length measures about 36 cm, and the average weight approximates 3500 g.
The cranial end of the neural tube closes by 38 days from the LMP, and the caudal end closes by 40 days. Hence, folic acid supplementation to prevent neural-tube defects must be in place before this point to be eicacious (Chap. 9, p. 169). The walls of the neural tube form the brain and spinal cord. The lumen becomes the ventricular system of the brain and the central canal of the spinal cord. During the sixth week, the cranial end of the neural tube forms three primary vesicles. In the seventh week, five secondary vesicles develop: the telencephalon-future cerebral hemispheres; diencephalon-thalami; mesencephalon-midbrain; metencephalon-pons and cerebellum; and myelencephalonmedulla. Meanwhile, lexures develop and fold the brain into its typical configuration. The end of the embryonic period signiies completion of primary and secondary neuralization.
At 3 to 4 months' gestation, neuronal proleration peaks.
As expected, disorders in this cerebral development phase profoundly worsen function (Volpe, 2008). Neuronal migration occurs almost simultaneously and peaks at 3 to 5 months. This process is characterized by movement of millions of neuronal cells from their ventricular and subventricular zones to areas of the brain in which they reside for life (Fig. 7-8). U pregulation of gene expression for neuronal migration has been described (Iruretagoyena, 2014). Noninvasive methods to study fetal neurodevelopment have also been reported (Goetzl, 2016).
As gestation progresses, the fetal brain appearance steadily changes. Thus, it is possible to identiY fetal age from its external appearance (Volpe, 2008). Neuronal proliferation and migration proceed along with gyral growth and maturation (see Fig. 7-8). Sequential maturation studies by Manganaro (2007) and Dubois (2014) and their colleagues have characterized the developing fetal brain image using magnetic resonance (MR) imaging. Other recent investigations that also used MR imaging have quantiied development of subcortical brain structures from 12 to 22 weeks (Meng, 2012).
Myelination of the ventral roots of the cerebrospinal nerves and brainstem begins at approximately 6 months, but most
A B c �. .�. ''' FIGURE 7-8 Neuronal proliferation and migration are complete at 20 to 24 weeks. During the second half of gestation, organizational events proceed with gyral formation and proliferation, diferentiation, and migration of cellular elements. Approximate gestational ages are shown. A. 20 weeks. B. 35 weeks. C.40 weeks.
myelination progresses after birth. This lack of myelin and incomplete skull ossiication permit fetal brain structure to be seen sonographically throughout gestation.