What Happens to the Baby in the First Trimester?

The first trimester of pregnancy is the first three months of pregnancy, or up until about Week 12. The baby grows the most during the first trimester, which includes the formation and development of the body's major organs, the heartbeat being able to be heard on a Doppler, external genitalia formed, and more.

The first trimester is the first third or three months of pregnancy, when the baby grows the fastest. 

The embryonic stage (when the baby is called an embryo) of pregnancy begins and runs from about the 5th to the 10th week of pregnancy. 

Following the embryonic stage, the fetal stage begins (when the baby is called a fetus), which runs from the 11th week of pregnancy until birth. By the end of the first trimester, the fetus can weigh about 0.5 to 1 ounce and measure an average of 3 to 4 inches in length.

Many things happen to the baby in the first trimester, which is a time of rapid development for the baby. 

From conception through the end of Week 4:

• The body’s major organs and systems start to form
• The eyes and ears begin to develop
• The neural tube (which becomes the brain and spinal cord), the heart and circulatory system, and the digestive system start to develop
• Limb buds, that will develop into arms and legs, form
• The heart starts beating

From the beginning of Week 5 to the end of Week 8: 

• The body’s major systems such as the circulatory, nervous, digestive, and urinary systems continue to develop and function
• The mouth develops tooth buds that become baby teeth
• The eyes, nose, mouth, and ears become more defined
• The arms and legs continue to develop and can be easily seen
• The fingers and toes are webbed, but are clearly identifiable
• The baby's heartbeat can be heard using a fetal Doppler ultrasound
• The bones begin to grow 
• The nose and jaws develop quickly
• The embryo starts to develop a human form, but head is larger in proportion to the rest of the body

From Week 9 through Week 12: 

• External genitalia are formed
• Fingernails and toenails emerge
• The arms and legs are fully formed
• Eyelids are formed
• The voice box (larynx) starts to form in the windpipe (trachea)