At the end of the fifteenth week, the weight of the fetus is already about 50-70 grams, its length is 14-15 cm. The limbs grow, they can outstrip the size of the head due to the intensive development of the bones of the legs and arms. The future baby begins to actively bend his fingers and elbows, a unique skin pattern appears on the palms and fingers, and the process of nail formation begins.
At this stage, the cardiovascular system is being improved. The number of heart beats per minute is twice the mother's and is 140-160. A tiny heart pumps 20 liters of blood per day, contributing to the intensive development of the body. The fetus actively grows veins and arteries, there is a general improvement of the circulatory system. Own systems of veins and arteries are formed in each internal organ - kidneys, lungs, heart, brain, etc. The fetus acquires an unusual red color instead of pink, because it has very thin skin, and the circulatory system, developing, affects its color.
The nervous system at this time is undergoing important stages of formation. The mass of the brain increases, its furrows and convolutions increase. All muscles, internal organs and bones are braided with nerve fibers. A strong and stable connection is being established between the central nervous system and the peripheral one. Impulses begin to flow in both directions: from the brain to the organs and back. An important system of "reverse response" is emerging for the human body.
If before the fifteenth week, red blood cells (erythrocytes) were produced by the gall sac and liver, then after this stage, this function begins to be performed by the red bone marrow located inside the bones.
In the future baby, it is already possible to determine the blood type and Rh factor. The laying of the group and Rhesus occurs at the time of conception of the baby, but this information is realized only at the fifteenth week. There is a formation of special antigen proteins that appear on the surface of erythrocytes.