From conception to birth, a human baby goes through many stages as it is formed in the womb. During the first couple of months after conception, it is called an embryo and goes through a few different stages of development.
After an egg is fertilised by a sperm cell during conception, the fertilised egg is called a zygote. The zygote makes it's way down the Fallopian tubes to the uterus where five to eight days after fertilisation it attaches to the lining of the uterus wall and begins the stage of blastocyst development. Blastocyst development is complete by day 10 or 12 after fertilisation, and the zygote then moves on to the embryo stage. The embryo develops until about the end of the eighth week of pregnancy, and then it is considered a foetus until the baby is born.
Significance of the Embryonic Stage
During the embryonic stage of development, several significant elements of development occur. It is during this time that the placenta first develops as well as the beginning of internal organ development and external body features.
It is also during the embryo stage that neural development begins. By the end of this stage, the embryo will be capable of some movement, the heart will begin beating and facial features will form in the early stages of development.
Around two to three weeks after fertilisation, the fertilised egg has implanted in the uterine wall and cells have begun to form an umbilical cord. Growth of the embryo during this time centres around an axis that will become the spine and spinal cord. The embryo also starts to become more elongated and take on more of a human shape as it begins forming organs such as the heart and gastrointestinal tract.
At this stage, the woman's body produces hormones to stop the menstrual cycle. The organs continue to develop amd the heart starts to beat and pump blood through the circulatory system. Some tissue formation begins that will develop into the spine and other bones, and limb buds appear where the arms and legs will eventually form.
By this time, the embryo is about a half inch long and has eyes, slits where the mouth and nose will form and even hands with discernible fingerprints. Most of the organs will complete development at this time except for the brain and spinal cord. Neural development will have advanced enough to make the embryo capable of some movement, but at this stage it is considered to have a "reptilian brain" with only the most primitive of brain functions. Hair starts to form during this stage and some of the facial features. The tail disappears and the embryo looks more like a human.
According to the Merck Manual, most birth defects are formed during the embryonic stage. Because this is when organs are formed, it is the time when the developing baby is most vulnerable to the effects of drugs, radiation and viruses. Women who are pregnant shouldn't take any drugs or be given any live vaccines during this time unless it is essential to protect their own health. Most miscarriages also occur during the embryonic stage. According to "Fetal Medicine: Basic Science and Clinical Practice," the risk of miscarriage is virtually complete by the end of the embryonic period. After that the risk of a miscarriage is only about 2.5 per cent.