Title: 23.4: Fetal Stage - Biology LibreTexts
Headings: 23.4: Fetal Stage
23.4: Fetal Stage
Hello, Baby
Defining the Fetal Stage
Fetal Development
Weeks 9 to 15
Weeks 16 to 26
Weeks 27 to 38
Fetal Circulation
Prenatal Circulation
Postnatal Circulation
Birth Weight
Viability of the Fetus
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Content: Weeks 27 to 38
During weeks 27 to 38 after fertilization, the bones of the fetus complete their development. The fetus also grows rapidly during these final weeks, and its body fat increases substantially. Its formerly wrinkled skin starts to plump out as layers of subcutaneous fat are deposited. Additional changes that occur in the fetus during weeks 27 to 38 include the following: The fetus’s head hair grows thicker and coarser while the lanugo is shed. The waxy vernix covering the fetus becomes thicker at first, but most of it will disappear by birth. In preparation for breathing after birth, the fetus will repeatedly mimic breathing by moving the diaphragm. By about week 32, the lungs are likely to be fully developed so the fetus can breathe on its own, should it be born this early. The fetus can not only hear and feel touch, but its eyes can now detect light. In fact, the pupils can constrict and dilate in response to light.