Final answer:
At 16 weeks gestation, the fetus experiences growth and development, with more discernible limb movements and the initial formation of skin protectants like vernix caseosa and lanugo. The fetus's internal organs, such as the lungs and heart, become more developed and functional. Around this time, the pregnant person may start to feel the fetus's movements, known as quickening.
Step-by-step explanation:
At approximately 16 weeks of gestation, significant fetal developments occur. The fetus begins to demonstrate more powerful limb movements, becoming palpable to the mother as she may feel the quickening or the initial flutters of fetal movement. Due to space restrictions in the uterus, these movements usually confine the fetus to a characteristic "fetal position," with arms crossed and legs bent at the knees.
During this stage, the fetus is coated with a waxy substance known as vernix caseosa, which serves to protect and moisturize the skin. It also may help with lubrication during the birthing process. Additionally, a fine, silky hair called lanugo grows on the skin, providing insulation; this hair is generally shed as the fetus grows further, particularly around weeks 17 to 20.
The fetus at this point measures approximately 4.5 inches and has distinct fingertips. By the 24th week, the fetus has developed enough that premature infants have a chance for survival outside of the womb, with organs like the lungs and heart formed to a partially functioning stage and the continued growth of the brain. The amniotic fluid that the fetus circulates by swallowing and producing urine reaches its peak volume at about 34 weeks gestation, when it measures close to 800 ml.