Fetal Period

The final stage of fetal development

The fetal period of prenatal development begins on the last day of the eighth week after fertilization and continues until birth. This stage is characterized by the exponential growth of the embryo, now called a fetus, and the development of fully differentiated organs and tissues.

Because the fetus already developed precursors of all major organs during the embryonic period of development, it is less susceptible to toxins and other environmental abnormalities during this period. Toxic exposures or nutritional deficiencies are more likely to cause physiological abnormalities than a miscarriage.


Weeks 10 to 12

Between week 10 and week 12, the head accounts for nearly half of the fetus' mass. Its eyelids close during this period and do not open again until around the 7th month of development in the womb. The ears begin to take their final shape and tooth buds appear. The fetus' genitals are clearly differentiated at this point and may even be visible via ultrasound as early as week 12 (although sex prediction ultrasonography typically takes place after week 16).

Weeks 13 to 16

Between weeks 13 and 16, the baby begins to move with regularity. Lanugo, a fine, downy hair, begins to grow on the head. Meconium, a thick, dark green waste substance, is produced in the fetus' bowels and remains there (in most cases) until after birth.

Weeks 17 to 20

Week 17 to week 20 is a period of rapid growth for the fetus. It begins to accumulate a layer of fat underneath its skin and starts to progress through somewhat regular intervals of wakefulness and sleep. Increased fetal movement is likely to become apparent, in the form of a fluttering sensation in the mother's abdomen. During this period, the sex of the baby can be discovered via ultrasonography.

Weeks 21 to 25

Between weeks 21and 25, the fetus becomes covered in lanugo. Eyelashes and fingernails begin to develop. Around this time, doctors are first able to hear the fetal heartbeat using a stethoscope. Movement continues to increase and the fetus develops a startle reflex.

Weeks 26 to 28

From week 26 to 28, the lungs are rapidly developing as the fetus readies for its first breath of air. By week 28, gas exchange is possible and the fetus may survive outside of the womb. The brain is also developing at a fast pace and the fetus' nervous system begins to control some of its bodily functions. Around week 28, the eyelids open for the first time since early in the embryonic period of development.

Weeks 29 to 36

Between weeks 29 and 35, the lungs continue to develop and the fetus puts on weight (much of it in the form of body fat). The fetus develops its first immunities, increasing the likelihood of survival should a premature birth take place (by week 36, an infant has a high probability of survival, provided it receives sufficient medical care). Around week 35, lanugo begins to be shed and may be replaced by vellus hair (colloquially known as "peach fuzz").

Weeks 37 to 40

By week 37, the fetus is considered to have been carried to its full term. It may have a full head of hair and its fingernails and toenails may extend past the ends of its fingers and toes. During this time, the fetus can gain as much as half a pound of weight every week.