Conception occurs when a sperm meets with and fertilizes an ovum, or egg, that is traveling down the fallopian tube. The result is a one-celled entity called a zygote. A zygote normally has 46 chromosomes; 23 from the mother and 23 from the father. Chromosomes are a fetus's genetic code. This genetic code will determine a baby's sex, eye color and even skin color and physical development. Fetal development during the first trimester is rapid and nonstop.
Zygote
Interestingly, a woman is technically not pregnant during the first two weeks that are included in the typical 40-week pregnancy. Conception occurs approximately two weeks after menstruation begins. These two weeks are counted as part of the pregnancy. During week three---the week after fertilization---the zygote begins to divide rapidly into a ball of multiple cells until it reaches the uterus. Once in the uterus it floats around for several days, continually dividing into more cells. According to the Mayo Clinic, the inner group of the zygote's cells will become the embryo from which the fetus's body will form, and the outer group of cells will eventually create the placenta. By week four the zygote is called a blastocyst and implants itself into the uterus, where it will remain until birth. It is at this stage that a pregnancy test will be positive because the secretion of the hormone beta human chorionic gonadotropin, or hCG, begins after implantation.
Embryo
Once implanted, the blastocyst is called an embryo. During week five of pregnancy the embryo grows rapidly; the brain, spinal cord and heart begin to develop. By week six the rudimentary heart is beating, and the spinal canal has formed. Facial features begin to take shape. Small arm and leg buds begin to form on the trunk of the body, and the ear canal is present. The face and brain continue to form, and tiny nostrils are visible by week seven. By the end of week seven, every necessary organ for life has begun to develop in the embryo, according to the American Pregnancy Association. This tiny embryo is still no bigger than an aspirin.
Weeks 8 to 12
As its arms and leg grow longer, the embryo begins to move at approximately week eight. These movements are still indiscernible to the mother. Fingers are beginning to form, as are the upper lip and nose. By week nine, arm bones have formed, and the embryo can bend the arms at the elbow. At week 11, the embryo officially becomes a fetus, according to the Mayo Clinic. The fetus's eyes are widely spaced, red blood cells begin to form in the liver and genitalia begin to differentiate. By 12 weeks, the fetus has the profile of a human face. The fetus will weighs half an ounce and be approximately 2.5 inches long by the end of the first trimester.
