Fetus/ˈfiːtəs/ Definition 1 of 2Pro Tip 1/3Stage Specific Use 'fetus' for development stages over eight weeks, before that it's called 'embryo'. The fetus is now the size of an orange. Pro Tip 2/3Medical Context 'Fetus' is often used by healthcare professionals to communicate about pregnancy. The fetus's heartbeat is strong and healthy. Pro Tip 3/3Technical Term 'Fetus' is a technical term; 'baby' might be used informally after birth. We saw our baby's fetus during the sonogram.