Within the first month of pregnancy a woman's egg becomes fertilized and undergoes rapid changes to eventually form what is called an embryo. Merck Manual explains that a fertilized egg (called a zygote) develops into a blastocyst, then an embryo and eventually a fetus in the later weeks of pregnancy. In the medical field the first month of pregnancy actually consists of two weeks that took place before an egg is even fertilized. The month begins on the first day of the last menstrual period. Two weeks after the menstrual period, a woman ovulates, or releases an egg for fertilization. Once the sperm penetrates the egg, a zygote forms, which is the first stage of embryo development.
In order for a zygote to form a woman's body must release an egg prior to intercourse. As soon as sperm penetrates the egg, the cell division begins, forming the zygote. Although the zygote is nothing more than a cluster of cells, it contains vital information, including the genetic details (DNA) of a child, explains the University of Maryland Medical Center (UMM). Half of the zygote's information comes from the mother's egg and the other half is from the father's sperm. For the next several days of it comes into existence the zygote will make its way down the fallopian tube. While traveling, the zygote cell divides rapidly until the ball of cells increases in size.
The rapid cell division the zygote undergoes turns it into a group of cells surrounded by an outer shell, called a bastocyst. The inner cells will soon become the embryo, surrounded by a protective membrane that will provide nourishment to the embryo and soon the fetus during the pregnancy. By about day five, the blastocyst reaches the woman's uterus (womb) and becomes attached to the lining on day six. This is called implantation. It can result in some mild to moderate cramping and a little spotting of blood from the vagina. The lining of the womb will provide nourishment via the mother's bloodstream, explains the UMM. Between the 10th and 12th day after conception several layers of membranes (called amnion) begin developing and will later form the amniotic sac. This sac will fill eventually fill with fluid and surround the embryo until delivery.
Cell division continues in the blastocyst. The cluster of cells is now called an embryo. New, specific cells are being formed now that serve a purpose, such as blood cells and nerve cells. Some of the baby's physical features will form and most of the organs will form now that the cells have become an embryo. Much of this organ development begins around the third week after fertilization. The embryo begins elongating taking on more human-like shape during this week. By about day 16 or 17 the brain and spinal cord areas will form. Before the first month has even ended, the heart may begin pumping fluid through the blood vessels.