Answer: During the first eight weeks, a fetus is called an embryo. The embryo develops rapidly and by the end of the first trimester, it becomes a fetus that is fully formed, weighing approximately 0.5 to 1 ounce and measuring, on average, 3 to 4 inches in length.
Through the first trimester, a baby goes from being a fertilized ovum to a fetus of about 6cm in length at 12 weeks. By the end of the first trimester, your baby's heart is starting to beat, and the brain, stomach and intestines are developing.
Explanation: hope this helps you!!!