Within three weeks, the blastocyte cells ultimately form a little ball, or an embryo, and the baby’s first nerve cells have already formed. Your developing baby is called an embryo from the moment of conception to the eighth week of pregnancy. After the eighth week and until the moment of birth, your developing baby is called a fetus. Stages of Growth: Month by Month.
As the fertilized egg grows, a water-tight sac forms around it, gradually filling with fluid. This is called the amniotic sac, and it helps cushion the growing embryo. The placenta also develops. The placenta is a round, flat organ that transfers nutrients from the mother to the baby, and transfers wastes from the baby. A primitive face will take form with large dark circles for eyes. The mouth, lower jaw, and throat are developing. Blood cells are taking shape, and circulation will begin.
Your baby’s facial features continue to develop. Each ear begins as a little fold of skin at the side of the head. Tiny buds that eventually grow into arms and legs are forming. Fingers, toes and eyes are also forming. The neural tube (brain, spinal cord and other neural tissue of the central nervous system) is well formed. The digestive tract and sensory organs begin to develop. Bone starts to replace cartilage. The head is large in proportion to the rest of the baby’s body. After the 8th week, your baby is called a fetus instead of an embryo.
Your baby’s arms, hands, fingers, feet, and toes are fully formed. Your baby can open and close its fists and mouth. Fingernails and toenails are beginning to develop and the external ears are formed. The beginnings of teeth are forming. Your baby’s reproductive organs also develop, but the baby’s gender is difficult to distinguish on ultrasound. By the end of the third month, your baby is fully formed. All the organs and extremities are present and will continue to mature in order to become functional. The circulatory and urinary systems are working and the liver produces bile. At the end of the third month, your baby is about 4 inches long and weighs about 1 ounce. Since your baby’s most critical development has taken place, your chance of miscarriage drops considerably after three months.