Final answer:
The fetus gains weight rapidly during the third trimester. Sex organs and the umbilical cord develop during the first trimester, whereas the lungs continue to develop and do not reach functional maturity until the third trimester.
Step-by-step explanation:
During the third trimester, the fetus rapidly gains weight. During the first trimester, the fetus’s sex organs develop. The umbilical cord forms during the first trimester. The lungs do not develop until the third trimester.
Significant weight gain occurs in the third trimester as the fetus grows to 3 to 4 kg (6 ½ -8 ½ lbs.) and about 50 cm (19-20 inches) long. Sex organs begin to differentiate around weeks 9 to 12, and are clearly visible by ultrasound, indicating that their development occurs during the first trimester. The umbilical cord, which is essential for delivering nutrients and oxygen to the fetus and removing waste, also forms in the first trimester. While the basic structures of the lungs are present from an early stage, the organs continue developing and do not reach a functional state until the third trimester, increasing the chances of survival if the baby is born prematurely at this stage.