Q&A: Fetus In First Trimester?
What basic changes does my baby go through in the first trimester?
In the first trimester (through week 13), baby starts off as a tiny ball of cells called a blastocyst, becomes an embryo when the bundle implants into the wall of your uterus, and officially reaches fetus status after eight weeks. The placenta forms to keep baby connected to all those nutrients you're trying to hold down, and baby starts growing into a, well, baby. In these few weeks, her major organs, nervous system, head, face, ears, arms, fingers, legs and toes form — maybe before you even start to show! Bones, lungs and hair also start to grow while temporary teeth buds develop and baby's heart begins beating.
American College of Obstetrics and Gynecologists. Your pregnancy and birth. 4th ed. Washington, DC: ACOG; 2005.