A typical pregnancy lasts 40 weeks from the first day of your last menstrual period (LMP) to the birth of the baby. It is divided into three stages, called trimesters: first trimester, second trimester, and third trimester. The fetus undergoes many changes throughout maturation. How Soon Can You Tell If You Are Pregnant?
The end of the first trimester is at about week 12, at this point in your baby’s development: The nerves and muscles begin to work together. Your baby can make a fist. The external sex organs show if your baby is a boy or girl. Eyelids close to protect the developing eyes. They will not open again until week 28.
Picture of First Trimester (4 Weeks) At 4 weeks gestational age, a baby in the first trimester starts to grow a brain, spinal cord, and heart. Arm and leg buds begin to form. The baby is considered an embryo at this point and is 1/25 of an inch long. Image Source: Copyright © Medical RF / Phototake — All rights reserved.