The brain itself has approximately the same overall structure as it will have at birth, although an enormous amount of refinement will take place in succeeding weeks and months. Several developments take place in the digestive system during week 10. The hard, bony part of the palate forms, and the thyroid, pancreas and gall bladder complete their formation. The intestines now move fully into the abdomen from the umbilical cord, and muscles in the walls of the digestive tract become functional. Fetal blood, which originally was formed in blood islands surrounding the early embryo, now begins to manufactured in the spleen and bone marrow.
Vocal cords form in the larynx, or voice box.
By the end of week 12 the baby , although perfectly formed, is only 3.5 inches long from crow to rump and weighs only one ounce. Week 12 ends the first trimester, the initial period of development. This period has largely been taken up with rapid development of the organ systems rather than with growth in size. The early weeks of pregnancy are a critical time, because during this period the various organ systems are being fundamentally elaborated. It is at this stage that they are most sensitive to environmental chemicals, drugs and viruses that can cause birth defects.