Embryo and fetus definitions

The term embryo refers to the early stage of development of a future baby, inside the uterus. It is called a fetus from the third month of pregnancy, once all its organs are formed.

Fertilization and cell development

The development begins with fertilization during sexual intercourse, where millions of sperm are released into the vagina. These travel through the cervix and into the fallopian tubes, where one sperm fuses with an egg to form a single cell with 46 chromosomes. This cell divides repeatedly, forming the basis of the baby's body and its supporting structures.

Implantation and early development

During the second week of pregnancy, the embryo implants into the thickened uterine lining. It then begins to grow rapidly, and by the 23rd day, the heart starts to beat. By the sixth week of amenorrhea, the fetus is about 5 millimeters long, and its heartbeats can be detected by ultrasound.

Early ultrasound observations

From the second week of pregnancy, the embryo can be seen on ultrasound as a small, haloed structure. By the fifth week of amenorrhea, the first heartbeats are visible. During the second month, the brain, ears, eyes, and limbs develop, and by the eighth week, the embryo is about 22 millimeters long with a formed brain and digestive system.

Transition to fetal period

From the third month of pregnancy, the term fetus is used. By the end of the second month, all organs are formed, and the fetus begins to grow in size and weight. At the end of the third month, it measures 12 centimeters and weighs 65 grams, with all major systems in place.

Development in the second trimester

By the 12th week of pregnancy, the fetus is about 4.5 centimeters long and weighs 10 grams. Its fingers and toes are visible, and its face begins to take shape. By the 14th week, the sex of the fetus may be identifiable. The fetus continues to grow and develop, with organs becoming more functional.

Growth and viability

By the 22nd week of pregnancy, the fetus weighs 500 grams and measures 26 centimeters. It is now considered viable, though its lungs are not fully developed. Doctors may use corticosteroids to help mature the lungs in case of preterm labor.

Fetal development and maturation

During the third trimester, the fetus continues to grow, gaining fat and developing reflexes. By the 41st week of pregnancy, the baby is ready for birth, though it is too large to be fully visualized on ultrasound. The placenta, umbilical cord, and amniotic fluid support the fetus throughout pregnancy.

Placental and umbilical functions

The placenta and umbilical cord are formed in the first trimester. They facilitate the exchange of nutrients, oxygen, and waste between the mother and fetus. The fetus does not breathe in the womb; instead, oxygen and carbon dioxide are exchanged through the placenta.