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Assessment of Fetal Growth and Development
The developing fetus is affected by social and environmental influences, including maternal nutritional status; substance use (both legal and illicit); and psychologic trauma. Correspondingly, the psychologic alterations experienced by the parents during the gestation profoundly impact the lives of all members of the family. Growing evidence implicates the importance of these and other maternal and paternal experiences that occur during and prior to the pregnancy (and even among members of earlier generations) on the subsequent development of the individual (epigenetic effects; see Chapter 100 ). The complex interplay among these forces and the somatic and neurologic transformations occurring in the fetus influence growth and behavior at birth, through infancy, and potentially throughout the individual's life.
Somatic Development
Embryonic Period
Table 20.1 lists milestones of prenatal development. By 6 days postconception age, as implantation begins, the embryo consists of a spherical mass of cells with a central cavity (the blastocyst ). By 2 wk, implantation is complete and the uteroplacental circulation has begun; the embryo has 2 distinct layers, endoderm and ectoderm , and the amnion has started to form. By 3 wk, the 3rd primary germ layer (mesoderm) has appeared, along with a primitive neural tube and blood vessels. Paired heart tubes have begun to pump.
Table 20.1
Milestones of Prenatal Development
| WK | DEVELOPMENTAL EVENTS |
|---|---|
| 1 | Fertilization and implantation; beginning of embryonic period |
| 2 | Endoderm and ectoderm appear (bilaminar embryo) |
| 3 | First missed menstrual period; mesoderm appears (trilaminar embryo); somites begin to form |
| 4 | Neural folds fuse; folding of embryo into human-like shape; arm and leg buds appear; crown-rump length 4-5 mm |
| 5 | Lens placodes, primitive mouth, digital rays on hands |
| 6 | Primitive nose, philtrum, primary palate |
| 7 | Eyelids begin; crown-rump length 2 cm |
| 8 | Ovaries and testes distinguishable |
| 9 | Fetal period begins; crown-rump length 5 cm; weight 8 g |
| 12 | External genitals distinguishable |
| 20 | Usual lower limit of viability; weight 460 g; length 19 cm |
| 25 | Third trimester begins; weight 900 g; length 24 cm |
| 28 | Eyes open; fetus turns head down; weight 1,000-1,300 g |
| 38 | Term |
During wk 4-8, lateral folding of the embryologic plate, followed by growth at the cranial and caudal ends and the budding of arms and legs, produces a human-like shape. Precursors of skeletal muscle and vertebrae (somites) appear, along with the branchial arches that will form the mandible, maxilla, palate, external ear, and other head and neck structures. Lens placodes appear, marking the site of future eyes; the brain grows rapidly. By the end of wk 8, as the embryonic period closes, the rudiments of all major organ systems have developed; the crown-rump length is 3 cm.
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