The development of a single cell into a small but identifiable human being in only 12 weeks is amazing to observe. The merging of two single cells, the sperm and the egg, starts a cascade of cell division, differentiation and growth that results in a fetus, a creature whose appearance is a far cry from the initial zygote. The developing human passes through three stages before becoming a fetus, a stage that eventually culminates in birth.
Zygote
The cell formed by the merger of the sperm and egg is called a zygote. The zygote contains 46 chromosomes, half from each parent cell, and will spend only a few days being classified as a zygote. The zygote divides rapidly as it journeys down the fallopian tube to the uterus, a trip that takes around 5 days.
Blastocyst
By the time the zygote reaches the uterus, it's transformed into a ball of cells called a blastocyst. Blastocysts are a group of cells clustered around a central cavity; the outer layer of cells is only one layer thick, except in the area called the fetal pole, which is three of four cells thick, the Merck Manual states. The fetal pole is the first part of the blastocyst to implant in the uterus, anywhere from day 5 to 8 post conception. Once the blastocyst implants, another layer of cells, known as the trophoblast, begins to grow around the embryo. The trophoblastic layers become the amniotic sac; once the sac has encircled the blastocyst, by around day 10 after conception, the blastocyst is known as an embryo.
Embryo
Since pregnancy is counted from the start of the last menstrual period, pregnancy weeks are 2 weeks beyond the actual gestation period, or the time measured from conception. This means the embryonic stage begins at the fourth pregnancy week. In the embryonic stage, cell differentiation rapidly occurs as the embryo differentiates into three layers, the ectoderm, or inner layer, endoderm, or outer layer and mesoderm, or middle layer, which will form different parts of the embryo in week five, the Mayo Clinic states. The rudimentary heart begins to beat and blood begins to form in weeks six to seven; limb buds are visible.
By week nine, all the major organs are forming, and toes, elbows, nipple and hair follicles are present. By week 10, the last week of the embryonic period, facial features start to take shape, the University of Maryland Medical Center explains. After this point, which is the eighth week from conception and the tenth week of pregnancy, the embryo is called a fetus.
Fetus
Fetal development for the last two weeks of the first trimester includes refinement of the structures already present and fetal growth. By week 12 of pregnancy, it's possible to tell the sex of the fetus. The eyelids fuse and remain closed until around week 28. The fetus can moves its long arms and legs, but the movement can't be felt yet, since the fetus is only 2.5-inches long and weighs only a few ounces, according to the Mayo Clinic.
