What Are the 3 Trimesters of Pregnancy?

There are three trimesters of pregnancy, each marked by certain fetal developments. The first trimester of pregnancy refers to the first three months (embryonic stage), the second trimester is weeks 13 through 26, and the third trimester is weeks 27 to 40. 

A pregnancy is considered full-term at 40 weeks. There are three trimesters of pregnancy, each marked by certain fetal developments. 

The First Trimester 

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

About 5 weeks after conception, when sperm fertilizes an 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. 

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, muscles, bones, nervous system, heart, face, arms and legs, and sexual organs begin 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 out 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.

In the second trimester of pregnancy, weeks 13 through 26, the baby continues to grow and develop rapidly. The mother may feel the baby's first fluttering movements between 16 to 20 weeks. The 20th week marks the halfway point of pregnancy. 

Fetal development during the second trimester includes:

• The placenta is fully developed
• The fetus kicks, moves, and can turn from side to side
• The fetus develops reflexes including swallowing and sucking
• The fetus can respond to certain stimuli
• The brain undergoes its most important period of growth from the fifth month on
• Hair grows on the head of the fetus
• Eyelids begin to open and eyebrows and eyelashes are visible
• The fetus develops all its organs and systems 
• By the end of the second trimester, a fetus will be about 13 to 16 inches long and weigh about 2 to 3 pounds

The mother will usually begin to feel better and the pregnancy will show more. 

In the third trimester, weeks 27 to 40, is the final stretch of pregnancy. The uterus expands from approximately 2 ounces before pregnancy to 2.5 pounds at the time of birth as the fetus continues to grow in size and weight. 

Fetal development during the third trimester includes:

• The brain continues to develop
• The fetus can see and hear
• The kidneys and lungs continue to mature
• The bones of the skull remain soft so the fetus can more easily pass through the birth canal
• A fetus can suck its thumb and is able to cry
• By week 36, the head may drop into the pelvic area (called lightening)
• By weeks 38 to 40 weeks, the fetus’ fine, soft hair on the body and limbs (lanugo) has disappeared almost completely
• By 38 to 40 weeks, the lungs have matured completely
• The head will usually turn downward during the last couple of weeks of pregnancy

By the end of the third trimester, a fetus is about 19 to 21 inches long and the average weight is 6 to 9 pounds.