As a subject matter expert in developmental biology, I can provide a comprehensive overview of the fetal phase, which is a critical period in human prenatal development. The fetal phase typically begins at the end of the embryonic period, around the end of the eighth week of gestation, and continues until birth. This phase is characterized by significant growth and the continued differentiation and maturation of the organs and systems that were initiated during the embryonic period.

The Fetal Period: Weeks 21 to Term

The fetal period is a time of remarkable transformation. The embryo, which has developed the basic outlines of its organs, transitions into a fetus. This period is marked by the completion of organ development and the growth of the fetus to a size and maturity level that is viable for life outside the womb.

**Introduction: Development from Embryo to Fetus**

The transition from an embryo to a fetus is not an abrupt one; it is a gradual process. During the embryonic period, the major structures of the body are established. However, as the embryonic period concludes, the developing human takes on a more recognizable form. The organs start to take on their definitive shapes and functions, although they are not yet fully mature.

Growth and Development

During the fetal period, the fetus experiences rapid growth. The weight increases from a few grams at the beginning of this phase to about 3.5 kilograms (about 7.7 pounds) at full term. The length also increases significantly, from a few centimeters to about 50 centimeters (20 inches) by the time of birth.

Organ Development and Maturation

The organs that were initiated during the embryonic period continue to mature. The brain develops further, with the formation of more complex neural networks. The heart becomes a fully functional organ, pumping blood through a now fully formed circulatory system. The lungs continue to develop, preparing for the first breaths of air after birth. The digestive system also matures, with the intestines and other organs preparing for the digestion of food.

Functional Development

In addition to the physical growth and maturation of organs, the fetus also begins to exhibit more advanced functions. The fetus starts to move more, and by the end of this period, the mother can often feel these movements. The fetus also develops the ability to hear, see, and taste, although these senses are not fully developed until after birth.

Preparation for Birth

Towards the end of the fetal period, the fetus undergoes a series of changes to prepare for birth. The position of the fetus in the womb often shifts as it gets ready for the journey through the birth canal. The lungs produce a substance called surfactant, which helps to prevent the alveoli from collapsing and is essential for breathing air after birth. The brain also continues to develop, with the formation of more complex cognitive functions.

Conclusion

The fetal phase is a time of incredible change and growth. It is during this period that the developing human moves from a stage of initial organ formation to a stage of preparation for life outside the womb. The fetus grows in size, the organs mature, and the fetus begins to exhibit more advanced functions, all in preparation for the moment of birth.

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The Fetal Period: Weeks 21 to Term. Introduction: development from embryo to fetus is not abrupt, but the embryo changes to a recognizable human being and develops all the basic outlines of its organs and is then called a fetus.read more >>