1-3 months of pregnancy: fetal tissue development 1-3 months of pregnancy is a critical period for the formation of fetal tissues. During this period, some of the fetus\’s basic structures have formed, including the head, body, limbs, and internal organs. At this time, the focus of the fetus is on its developing cells. The following is a picture of fetal development in 1-3 months of pregnancy: In 1-2 months of pregnancy, the outline of the fetal head develops from the future spine. Important developmental stages include the formation of the embryonic central nervous system, circulatory system, urinary system, gastrointestinal tract, and penis. within three months