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.
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.
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.