The first three months of pregnancy have the most developmental changes of the embryo. The first seven days after fertilization, the fertilized egg moves its way and implants in the uterus. After implantation, the life journey of an embryo begins. From there the thickness of endometrium or inner lining of the uterus is maintained by progesterone from the secretions of corpus luteum.
From the third day of implantation, the yolk sac (amniotic sac) begins to develop together with the tissues that form the placenta and umbilical cord. By the start of first week, the embryo already has an ectoderm. The ectoderm is the primary formation of nerves, the brain and the skin. The mesoderm forms the bones, muscles, the heart, blood vessels, and genital organs. The endoderm forms the liver, the stomach, the lungs, the intestines and the urinary tract.
By the beginning of organogenesis, the spinal cord is developed. Part of the lumbar area starts to form by the start of one and a half month. In a 3D ultrasound, you will see an embryo, like a tadpole. The head is bigger than the extremities. Other facial features begin to form like the eyes, mouth, nose and limb. The heart begins to pump.
By the second month, fetus engages in major developmental stage. It is here where the major organs are fully formed. The second month is somewhat remarkable because it is in this month that bone cells appears and the fingers and toes are now visible. The ears also begin to form.
When the fetus reaches to three months old gestation, the external genitals are formed. It is on the third month that you can already differentiate the gender of a fetus. The heart beats up to 160 in one minute. The kidneys began to function. The umbilical cord is now fully developed. And by the end of three months gestation, the fetus began to move.