umbilical vein: a vein in the umbilical cord; returns nutrient blood from the placenta to the fetus
entity: that which is perceived or known or inferred to have its own distinct existence (living or nonliving)
menstrual phase: the phase of the menstrual cycle during which the lining of the uterus is shed (the first day of menstrual flow is considered day 1 of the menstrual cycle)