fetusS

Fetus is the term used in human development for the developing organism from roughly the ninth week after fertilization until birth. The earlier stage, from fertilization through about eight weeks, is called the embryo. Because pregnancy age is typically counted from the last menstrual period, the fetal stage begins in the late first trimester and continues to delivery.

During the fetal period, organ systems mature and growth accelerates. The fetus gains size and weight, the skeleton develops, and the lungs, brain, and other organs undergo functional maturation. Movements begin in the second trimester and can be detected by the mother or seen on ultrasound. Sensory systems and reflexes develop as term approaches. The fetus relies on the placenta and umbilical cord for oxygen and nutrients and for waste removal.

Prenatal assessment uses ultrasound to monitor growth and anatomy, along with fetal heart-rate checks and, when indicated, genetic testing such as noninvasive screening or amniocentesis. Viability outside the womb increases with gestational age and medical care; contemporary medicine has enabled survival for many babies born in the mid- to late-tentieth week, though outcomes vary widely by gestational age and condition at birth. Prenatal care generally emphasizes maternal health, nutrition (including folic acid), avoidance of harmful exposures, vaccination, and management of chronic diseases to support fetal development.

Terminology notes: the standard English terms are fetus (singular) and fetuses (plural). The spelling fetuss is nonstandard in medical usage.