An embryo is the term used for a developing human from the time of fertilization through the end of the eighth week of gestational age, after which it is called a fetus. On transvaginal ultrasound, the embryo first becomes visible within the gestational sac at approximately 5 to 6 weeks of gestational age, initially appearing as a small, echogenic structure adjacent to the yolk sac. By 6 to 7 weeks, cardiac activity can usually be detected, and by 8 weeks, early limb buds and head structures begin to become visible. Documenting the presence, size, and cardiac activity of the embryo is a fundamental part of first trimester ultrasound and helps confirm viability and establish an accurate gestational age.
Sonography Term