From the joining of two cells, through stages of tremendous growth and development, to the birth of your baby, pregnancy is a remarkable time. Follow along the stages of your baby's development, phase by phase, with this informational series from Mike Samuels, MD, and Nancy Samuels, authors of The New Well Pregnancy Book.


10-12 Weeks
By 10 weeks, connections between muscles and nerves in the peripheral nervous system are mature enough to transmit messages to and from the brain. The skin over most of the body is now responsive; that is, touching it will cause the fetus to move. 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. Already the fetus shows certain specific reflexes: it opens its mouth when its face is touched and bends its fingers when its arms are touched.

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.

During week 11, the last of the twenty deciduous tooth boods and their sockets form in the jaw. Vocal cords form in the larynx, or voice box. Several of the digestive organs become functional: the liver begins to secrete bile, the pancreas begins to produce insulin, and the intestines form into folds lined with villi and intestinal glands. Further maturation takes place in the digestive system during week 12: taste buds from on the tongue, and the salivary glands become functional. The musculature and the neural system mature, making possible much more purposeful movements: breathing, sucking, and swallowing motions begin, and the fetus starts to oppose its thumb to its other fingers.

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.