Theoretically, no growth of the embryo takes place until a week has passed from the time the egg has been fertilized. When fertilization takes place, the fertilized egg comes to be known as a zygote. Inside this zygote is the complete DNA material required by it to create a baby. The zygote swims along the fallopian tubes, keeping on reproducing new cells even as it makes its way.

Now, these cells attach at the ends, forming a circle, which permits the development of cells on the exterior as well as the interior of the circle. In case the zygote is to be divided into two, therefore permitting the growth of twins, this division normally takes place before the embryo stage of pregnancy starts. It is only when the zygote fastens itself to the lining of the uterus, does the embryo stage of pregnancy formally begin.

The embryo stage of the pregnancy lasts until eight weeks at which point the embryo formally changes into the fetus.

Now, the process of cell differentiation is initiated by the embryo. This differentiation permits the cells to gradually form into individual organs, such as the heart, liver and skin, and also forming the new growth that will in the course of time become the head, legs, and arms of the fetus. In case any critical complications occur, due to any reason, the embryo tends to be immediately terminated, resulting in a miscarriage.

A number of miscarriages take place in the first three months of pregnancy and most of them happen in this embryo stage of the pregnancy, since the DNA identifies that there are complications with its development.

During the embryo stage of the pregnancy, the cellular differentiation starts to function on the circular structure. The external rim of this circular forms the initial phases of the spinal cord and the head. Now, as the embryo grows, this round shape becomes vague and indefinite while the other cells start to develop further parts of the growing embryo. A great part of this development happens in a short time span, bearing in mind the extent of differentiation required.

In the initial weeks of the embryo stage of the pregnancy, a woman may not be aware that she is with child. The embryo is extremely minute, not more than one-fifth of an inch in length and no differentiation in cells has occurred, and therefore the woman's body might not respond markedly to the embryo. Once the cells start to differentiate, the embryo commences to make its own hormones.

Now, these hormones, together with the hormones of the pregnant woman, can bring about mood changes or cravings for food, which the women finds very unusual to experience. With the further development of the embryo, the woman's body has to fine-tune itself to the embryo's presence. The adjustments could produce morning sickness. Now, the expectant woman's body usually finishes making the pregnancy adjustments by the early part of the subsequent development phase that is the fetal stage.