Overview of Human Embryo Development

The term embryo is derived from a Greek word which means “That which grows”. An embryo is defined as “The developing organism from fertilization to the end of the eighth week”. The development process of an embryo in its mother’s womb is a wonder of nature. 

After the sperm is released inside the vagina, it travels through the cervix into the Fallopian tubes. Fertilization occurs when the sperm penetrates the egg. The fertilized egg is called zygote. It contains the genetic information (DNA) from the mother’s egg and the father’s sperm. 

During the next few days, the zygote multiplies itself while traveling down the Fallopian tube. The resultant group of cells is covered by a shell. This group of cells reaches the womb and sticks into the uterine wall. It receives nourishment from the mother’s bloodstream. The group of cells becomes the embryo. 

Gestation is defined as “The period of time from conception to birth”. During the third week of gestation, the embryo develops a distinct shape. Brain, spinal chord and heart begin to develop inside the embryo. The mother has to be very careful during this stage of pregnancy. 

The embryo develops significantly during the fourth week of gestation. The brain and the nervous system develop at a rapid pace. Optic vesicles develop on the sides of the head. The heart starts beating at a regular rhythm. Digestive and respiratory systems are formed. Arm and leg buds appear during this period. Blood starts circulating in the main vessels. 

During the fifth week the umbilical cord appears. It connects the mother to the baby. The baby receives oxygen and nourishment from its mother through this. It also facilitates the disposal of waste from the baby. Lungs are formed at this stage. The baby’s face takes a distinct shape. Mouth, nostrils, eyes, tongue and ears become distinctive. The trunk straightens out. Toes and elbows become visible. 

The fingers develop during the sixth week of gestation. Flexibility of the limbs improves. Eyes continue to develop. Intestines grow at this stage. A bud appears in the area of the genitals. However it is not possible to identify the sex at this stage. During the seventh week of gestation, the tail at the bottom of the baby’s spinal cord disappears. The baby’s head and other organs continue to grow. Hair and nipple follicles start forming. Testes and ovaries appear at this stage. Some movements may also be observed. 

The intestinal rotation begins during the eighth week of gestation. The eyelids and the ears develop at this stage. It acquires a human appearance. The baby is called an embryo till the end of the eighth week. From the ninth week it will be considered a fetus. Childbirth usually occurs sometime during the 38th week of gestation.