Fetalperiode

The fetalperiode, or fetal period, is the stage of prenatal development that follows the embryonic period and continues until birth. In humans it begins around the ninth week of gestation and extends to term. During this period the fetus undergoes rapid growth and most body systems mature and become functional, preparing the infant for life outside the womb.

Key processes of the fetal period include rapid physical growth, increasing length and weight, and the maturation of organs and organ systems. The circulatory and nervous systems become increasingly complex, the lungs undergo structural and biochemical maturation, and the digestive and metabolic systems begin to function more independently. The skin changes from translucent to thicker, and body fat accumulates to help regulate temperature after birth. External features become more defined, and by mid to late pregnancy the fetus develops hair, vernix caseosa, and lanugo.

Milestones vary by gestational age. In early fetal life, organs continue to differentiate and refine their functions; the heart beats steadily and fetal movements occur, although they are not always felt by the mother. In the second trimester, growth accelerates, sensory organs mature, and fetal activity becomes more noticeable. In the third trimester, lung surfactant production increases, brain development continues, and fat stores expand toward a viable birth weight. Viability generally improves as lung and other organ systems reach sufficient maturity.

Placental function and amniotic fluid support fetal development throughout the period, supplying nutrients and enabling growth and movement. Prenatal monitoring—including ultrasound and fetal heart rate assessment—helps track growth and detect potential complications such as fetal growth restriction or preterm birth.