The cells are multiplying rapidly and your baby now is made up of three specialized layers: the endoderm, mesoderm and ectoderm. The endoderm is the inner layer which will later develop into the digestive and respiratory organs, including the liver and lungs. The mesoderm is the middle layer which will soon become bones, kidneys, muscles, the sex organs and also the heart. The ectoderm is the outer layer and will make up tissue (such as skin and hair) and organs including the nervous system. The ectoderm also will form your baby’s umbilical cord. By the end of this week, the amniotic sac is starting to form. It will soon be filled with amniotic fluid, which your growing baby will float in.
By the tail end of this forth week of pregnancy, the earliest visible sac can be detected by ultrasound in close to all normal pregnancies, although ultrasounds are not routinely done during the first several weeks of pregnancy. Early ultrasounds (done during the first trimester) are done using a trans-vaginal probe which is placed directly into the vagina. This allows the transducer to get closer to your uterus and create better images, since your baby is so very tiny.