​which event is characteristic of the period of the fetus
Fetus A fetus is a stage in the prenatal development of viviparous organisms. In human development, a fetus or foetus (/ˈfiːtəs/; plural fetuses or foetuses) is a prenatal human between the embryonic state and birth. The fetal stage of development tends to be taken as beginning at the gestational age of eleven weeks, i.e. nine weeks after fertilization.[1][2] In biological terms, however, prenatal development is a continuum, with no clear defining feature distinguishing an embryo from a fetus. The use of the term "fetus" generally implies that an embryo has developed to the point of being recognizable as a human; this is the point usually taken to be the ninth week after fertilization. A fetus is also characterized by the presence of all the major body organs, though they will not yet be fully developed and functional and some not yet situated in their final anatomical location.
Obstetric ultrasonography A gestational sac can be reliably seen on transvaginal ultrasound by 5 weeks' gestational age (approximately 3 weeks after ovulation).The embryo should be seen by the time the gestational sac measures 20 mm, about five-and-a-half weeks. The heartbeat is usually seen on transvaginal ultrasound by the time the embryo measures 5 mm, but may not be visible until the embryo reaches 7 mm, around 7 weeks' gestational age.[5][10][11] Coincidentally, most miscarriages also happen by 7 weeks' gestation. The rate of miscarriage, especially threatened miscarriage, drops significantly if normal heartbeat is detected.[12]