Your body goes through many changes throughout the first trimester, as does your baby. Even in the first few weeks after conception, the fetus begins growing at a rapid pace, magically transforming from a beating heart and formed organs to developing arms, legs, and facial features. Here are some things that will happen as your baby goes through the first 12 weeks of life.

Weeks 1-4

Your baby will go through a lot of changes and developments during the first few weeks of pregnancy. Keep in mind that developments happen to start in week 1, though you won’t know you are pregnant until around week 3-4. During the first month of pregnancy, your baby is about 7mm long, which is roughly the size of a grain of rice. The baby is protected by the amniotic sac and begins forming its major organs and body systems. The heart will beat in the first month, the eyes and ears begin to develop, and the fetus will have limb buds that eventually become its arms and legs.

Weeks 5-8

During weeks 5-8, the facial features of your baby will keep developing, including its ears, eyes, nose, and mouth. Their buds begin turning into arms and legs, and their fingers and toes also develop during month two. The neural tube that includes the central nervous system, spinal cord, and brain will be formed during month two. While the fingers and toes are webbed, they are still noticeably and distinguished during month two. The bones, nose, and jaw are also starting to develop. The cells where the intestines, bladder, and lungs will be are formed as well.

Weeks 9-12

Your baby will be fully formed by the time the twelfth week of pregnancy is over. You may also start noticing some of the symptoms of pregnancy (if you haven’t already), like tender breasts, fatigue, and nausea. These symptoms will likely subside at the end of the first trimester. Your baby goes through a lot of development during the third month. They will have fingernails and toenails, eyelids, external genital organs, fetal movement, fully formed arms and legs, and their larynx will start forming in the trachea. By the end of the first trimester, your baby will be about 2 ½ inches long and weigh approximately an ounce.

As you can see, your baby goes through dozens of changes during the first trimester. While your baby is essentially fully formed, he or she is not fully developed and cannot survive independently. The first trimester is crucial for your baby’s health and survival, so it is important to keep yourself healthy during this time.