At what stage does the fetus primarily begin to grow and gain weight substantially?

The fetus primarily begins to grow and gain weight substantially during the fetal stage of development, which occurs from approximately the ninth week of pregnancy until birth. This stage is characterized by significant physical development, where the organs and systems that were formed during the embryonic stage continue to mature, and the fetus begins to accumulate body mass.

During this period, the cellular differentiation that occurred previously enables the fetus to develop more complex structures, and rapid growth takes place. By the end of the fetal stage, the fetus can be viable outside of the womb, showcasing the importance of this phase in preparing for birth.

In contrast, the germinal stage is primarily focused on the formation of a zygote and early cell division, the embryonic stage involves the critical development of major organs and structures but does not emphasize significant weight gain. The postnatal period refers to development after birth, which is not applicable in this context of fetal growth.