Between the 9th and 12th week of the fetal stage, the sex organs begin to differentiate. By the 16th week, the fetus grows to about 4.5 inches in length, and its fingers and toes become fully developed, complete with visible fingerprints.

Fetal Development Milestones (9-16 Weeks)

A period of significant neurological development occurs during the fetal stage, particularly between weeks 16 and 28, when the brain's size nearly doubles.

Fetal Brain Growth (Weeks 16-28)

Around the 36th week, a fetus is nearly ready for birth, weighing approximately 6 pounds and measuring about 18.5 inches. By week 37, all organ systems are developed enough for the fetus to survive outside the uterus with minimal risks associated with premature birth. The fetus continues to grow until about week 40, at which point movement becomes restricted and birth is imminent.

Late-Stage Fetal Development and Viability (36-40 Weeks)

At approximately 20 weeks of gestation, the fetus's sense of hearing begins to develop, marking a key sensory milestone.

Fetal Hearing Development (20 Weeks)

By the 32nd week of the fetal stage, the skeletal system has matured to the point where the bones are considered fully developed.

Fetal Bone Development (32 Weeks)

At 36 weeks of gestation, the muscles of the fetus are fully developed, a crucial step in preparing the fetus for life outside the womb.

Fetal Muscle Development (36 Weeks)

Around 40 weeks of gestation, the fetus is considered to have reached full-term development, signifying that it is fully prepared for birth.

Full-Term Fetal Development (40 Weeks)

By the 24th week of gestation, the fetus's lungs begin to develop. At this stage, its hearing is developed, allowing it to respond to sounds. The internal organs, including the heart, stomach, and intestines, are sufficiently formed to give a prematurely born fetus a chance of survival outside the womb. The fetus typically weighs up to 1.4 pounds at this point.

Fetal Development at 24 Weeks

A developing human is considered to be in the final stage of prenatal development from the 9th week after conception until birth. Which of the following best distinguishes the primary characteristic of this specific stage from the stages that precede it?

The fetal stage is the third and final phase of prenatal development, commencing at the ninth week and extending until birth. This period is primarily characterized by the rapid growth of the fetus's body, brain, and organs as they mature in preparation for life outside the womb.

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There are three stages of prenatal development:
1. Germinal Stage
2. Embryonic Stage
3. Fetal Stage 

Stages of Prenatal Development 

Spielman, R. M., Jenkins, W. J., & Lovett, M. D. (2020, April 22). Psychology (2nd ed.). OpenStax. https://openstax.org/books/psychology-2e/pages/1-introduction

OpenStax Psychology (2nd ed.) Textbook

The embryonic stage, lasting for about six weeks after the zygote implants in the uterus, is the second phase of prenatal development. During this critical period, the embryo's body and organ structures begin to form. A key development is the formation of the neural tube, which serves as the precursor to the brain and spinal cord.

Embryonic Stage

Fetal Stage

The germinal stage, lasting from conception through the first two weeks of prenatal development, is characterized by conception and rapid cell division through mitosis. During this fragile period, the mass of cells has not yet attached to the uterine lining, and fewer than half of all zygotes survive this initial stage.

Germinal Stage

An expectant mother learns that she was unknowingly exposed to a harmful substance approximately four weeks after conception. This substance is known to cause severe defects in the formation of the heart, limbs, and central nervous system. Based on the timing of the exposure and the types of defects mentioned, identify the stage of prenatal development during which the developing organism was most vulnerable. Justify your answer by explaining the key developmental processes occurring during this stage.

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