During which time in pregnancy does the fetus show a marked increase in size?

The correct answer is that the marked increase in size of the fetus occurs during the third trimester. This period is characterized by rapid growth and significant development as the fetus prepares for birth. In the third trimester, a number of important changes take place: the fetus gains weight as fat accumulates, organs mature, and the bones continue to harden. This explosive growth is crucial for developing the body systems that will function outside the womb.

During the second trimester, while there is growth and development, it is in the third trimester that the fetus undergoes substantial size increases. This is the time when many expectant mothers notice a significant increase in their belly size as well, reflecting the growth of the fetus.

The end of the first trimester marks the transition into the second trimester, where the fetus is still relatively small. Thus, while growth begins to accelerate at this point, it is not as pronounced as in later weeks. The assertion that no difference is observed is inaccurate, as fetal growth is progressive throughout pregnancy, with the most dramatic changes occurring in the final trimester.