The first weeks after fertilization
The embryonic period of fetal development covers the period of time from fertilization to the end of the eighth week of gestation (around 56 days). It has been described as the most critical period of development because it includes the moments when life is formed and all major bodily systems begin to develop, as well as the moments when life is most susceptible to infections, nutritional deficiencies and toxins.
Though the embryonic period is not said to commence until the moment of fertilization, the development of an embryo actually begins before conception. Most adult males produce thousands of spermatozoa every second and it takes about a month for each spermatozoon to travel from the testicles to a reservoir within the prostate gland. Each spermatozoon contains half of the human chromosomes needed to form a fetus.
At the same time as spermatozoa are preparing to exit the male body via ejaculation, the female body is preparing to receive them. About a day before conception, the woman's body produces a single mature ovum which, like the spermatozoa, contains half of the chromosomes necessary for human life. The ovum travels down a fallopian tube towards the uterus.
Following ejaculation inside the vagina, one spermatozoon penetrates the ovum inside a fallopian tube and fertilizes it, creating a zygote. This new creation contains a complete set of human chromosomes and an entirely unique DNA structure. Some consider this precise moment to represent the beginning of life.
Over the course of the next few days, the zygote will continue to travel down the fallopian tube toward the uterus. During this time, it divides numerous times and becomes a bundle of cells called a morula. After about five days of dividing, the morula has reached the uterus. An empty cavity has formed between the cells that make up the morula. The grouping of cells can now be called a blastocyst. The cells on the outside of the blastocyst will eventually become the external structures that support the embryo, such as the placenta. The inner cells will become the embryo itself. Together, the embryo and its supporting structures are called the conceptus.
About eight days after the ovum is fertilized, the conceptus implants in the lining of the uterus. It then begins a process called embryogenesis or organogenesis. The embryo's primary features start to take form during this period of rapid embryonic development. Different embryonic cells are produced, including blood cells and nerve cells. Genetic "mistakes" are not uncommon during this time and they can result in a miscarriage. Additionally, the embryo is at risk of being damaged by toxins, such as alcohol and other drugs, nutritional deficiencies and infections.
Weeks 3 to 8
In the third week of embryogenesis, the embryo begins to develop a brain, heart and spinal cord. The beginnings of a gastrointestinal system first appear. In weeks four and five, arms and legs first begin to bud, as do the structures that will eventually become eyes and ears. Brain development continues, the heart beats at regular intervals and the first bones begin to form. In week seven, all essential organs have begun their development and fingers, toes, nipples and hair follicles begin to form. In week eight, the ears and facial features are clearly visible. In most cases, the fetus now measures in at about an inch in length and weighs under an ounce. Early limb movements and brain waves can be recorded at this time.
The embryonic stage of fetal development ceases at eight weeks from the moment of fertilization. At this point, embryogenesis is complete and the embryo has developed into an organism that we begin to call a fetus. The next stage of fetal development, the fetal period, is ready to begin.