Final answer:
During weeks 16-20 of pregnancy, the fetus experiences quickening, adopts the fetal position, and develops a protective layer of vernix caseosa and a coating of lanugo, which is shed later in development.
Step-by-step explanation:
Between approximately weeks 16-20 of pregnancy, a fetus undergoes significant development. One notable change is the onset of quickening, which refers to the pregnant person beginning to feel the fetus's movements. Due to the limited space within the uterus, these movements typically settle the fetus into the typical 'fetal position,' characterized by crossed arms and legs bent at the knees.
The fetus's skin is protected and moisturized by a layer of vernix caseosa, a waxy substance produced by sebaceous glands. This coating also aids in providing lubrication to facilitate childbirth. Additionally, during this period, the fetus is covered in a fine hair called lanugo, which is believed to be important for regulating body temperature but is later shed as the fetus grows. In cases of extreme prematurity, infants may still retain lanugo.