This two-week difference causes the pregnancy to be calculated as two weeks longer than the age of the child. The embryo descends the fallopian tube and enters the uterus about three to four days after conception. The uterine lining thickens each month in preparation to receive the fertilized egg. If no fertilized egg implants itself, the uterus lining thins. This is the cause of the menstrual period of the woman.
From the 300 million sperm released, only about 300 reach the unfertilized egg. Of these, only one will fertilize the egg. For a brief moment, the shell of the egg is penetrable and the sperm can enter the egg. Once the sperm is inside the egg, the shell immediately hardens to prevent other sperm from entering. When the egg and sperm genes combine many characteristics are set, especially whether a boy or girl will form. Then the cell begins to divide rapidly. After successful implantation, the hormone HCG is released from the uterus into the body signaling a pregnancy. HCG is the hormone that is the basis of a pregnancy test.
Now the placenta begins to develop. A few cells penetrate deeper into the uterine lining. After only 21 days, the embryo has an s-shaped heart that is already beating. However, it is not yet divided into atrium and ventricle chambers. The internal organs such as the liver, lungs, stomach, intestines and the kidneys are beginning to form. It is at this point that the mother learns of her pregnancy when she misses her period. The embryo is now about 2.5 mm long.
All of the organs form during the 4th to 8th week of life. During this period, the nervous system, the eyes, the inner ears, the epidermis and the digestive tract form. The respiratory organs – including the associated glands and parts of the throat, more bones, muscles, heart, blood and lymph nodes, and much of the urinary and genital organs also form. The child now only has to grow until its birth.
Arms and legs become recognizable. Eyes and ears can be seen, the mouth opens and the tongue is beginning to form. The embryo is now about 0.6 cm long.
The heartbeats can be seen on an electric cardiogram. The heart is pumping the child’s blood at about 140 to 150 beats per minute through its body. The windpipe and the pancreas are now recognizable. The embryo is now about 1 cm long.
The child is now becoming aware of its environment, its position in the womb, the pressure on its body and temperature differences. With an electroencephalography (EEG) we can now measure brainwaves. The cartilage skeletal structures are formed and the formation of the musculature structure is well advanced. Every minute 100,000 new nerve cells are created. The embryo is now about 1.5 cm long.
All internal organs are formed. Nothing new is being developed. Now the embryo only needs food and time in order to grow and mature. In the jaw, buds form for the baby teeth. The basis of vision is set when the retina is connected to the brain. The sense of balance in the ear is completely formed and even drops of urine are excreted by the kidneys. The embryo is now about 2 cm long.
The infant now has unique fingerprints that will remain for life. The infant makes breathing movements and can swallow the amniotic fluid. On the palm of the infant’s hands, soft hand lines become visible. The cartilage starts to harden and become bone. The skeletal bones will only reach their final form many years later when the infant becomes an adult. The embryo is now about 4 cm long. From this point on the embryo is medically called a fetus.