What Is Considered the First Trimester?

The first trimester of pregnancy lasts roughly three months. During this time, the baby is the smallest and yet grows the most. 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.

The first trimester of pregnancy refers to the first three months and the time in which the baby grows the fastest. 

About five weeks after conception, when a man’s sperm fertilizes a woman’s egg, 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. 

What Happens in the First Trimester?

During the embryonic stage a number of changes occur:

• The cells of the embryo (embryonic stem cells) multiply rapidly and develop into the hundreds of different types of cells needed to form a human body
• The baby’s major organs, bones, muscles, nervous system, heart, arms and legs, face, and sexual organs start to form
• The placenta forms
  • Nutrients, oxygen, and water are drawn by the placenta from the mother’s blood and passed along to the baby through the umbilical cord
  • The placenta removes waste from the baby and filters our harmful substances from the mother’s body 
• The amniotic sac forms, which surrounds and protects the baby in the uterus
• At the end of the embryonic stage (about week 10 of the pregnancy) the baby will be about 1 inch long
• 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.