- The embryonic period begins at fertilization, 12 to 24 hours after ovulation, which is the release of an ovum into a woman's fallopian tubes. Fertilization, or conception, usually occurs in the fallopian tube, as one sperm cell penetrates the ovum to combine its 23 chromosomes with 23 present within the ovum. At this point, a unique being, called a zygote, with 46 chromosome is formed and development begins. As the zygote travels down the fallopian tube towards the uterus, its one cell will divide into two, which will divide into four, continuing to divide every 12 to 20 hours. After about three days of development, the zygote has divided into 16 cells, and has generally reached the entrance of the uterus, a stage that is called pre-embryo, or a 16 cell morula.
- Approximately five days after conception, a cavity appears at the center of the morula. This is considered the beginning of the blastocyst stage. During this stage, the blastocyst has an inner group of cells that will become the fetus, and an outer shell of cells that will become the placenta. It is during this stage that implantation will occur, the blastocyst fully attaching itself to the endometrium, which is the inside wall of the uterus, by about nine to 10 days after conception. Around the 12th day after conception, the blastocyst will begin producing hormones that can be detected in the urine of its mother, which can be detected by a pregnancy test. Thirteen to 14 days after conception, a "primitive streak" develops, which is the precursor for the growth of the central nervous system, and the pre-embryo has now graduated to embryo status.
- Three weeks after conception, the embryo is approximately 1/12 of an inch long, and is long and thin, somewhat resembling a worm. Around 18 to 21 days after conception, its heart will begin to beat. By the fourth week, the embryo will have reached an approximate length of 1/5 of an inch, and the head will begin to develop. At this stage, the embryo resembles a tadpole, with a tail and structures that resemble gills in the area that will develop into a throat.
- By the fifth week, arm and leg buds have developed, and hands with webbed fingers have formed. Facial features have also begun to form, and the embryo still bears a tail. By six weeks, the embryo is about half an inch long, and facial features have become more distinct, with slits forming where the nose and mouth will develop.
- During the seventh and eighth week after conception, the embryo will lose its tail, and the brain will begin to form. The embryo will have developed enough neural function to make it capable of some movement, and pain sensors appear. Facial features will become more refined, and hair will begin to form. After the eighth week, the developing child graduates from the embryonic period and is considered a fetus.
- During the embryonic stage, most of the organs of the body are formed, making this stage the most vulnerable one for the developing child. This period is when most birth defects occur, as the critical stages of development are influenced by factors such as nutritional deficiencies, drug exposure, alcohol consumption, environmental contaminants or viruses.
