Fetal development begins the moment the egg and sperm meet. These early days and weeks form the foundation for the unborn baby, who will change exponentially in the months from conception until birth. Early fetal development encompasses the first eight weeks of pregnancy, during which many of the most dramatic transformations occur. After week eight, the baby graduates from being an embryo to a full-fledged fetus.
Gestational v. Fetal age
Terminology with regard to pregnancy dates and fetal age often cause confusion. The medical community often refers to "gestational age" as the age of pregnancy from the last menstrual period or LMP. According to the American Pregnancy website, "fetal age" is the actual age of the growing baby from the time of conception. The difference between gestational versus fetal age is approximately two weeks.
Conception and The First Two Weeks
Fetal development begins at conception. During conception, the sperm penetrates an egg to form a set of 46 chromosomes, according to AmericanPregnancy.org. The fertilized egg then spends its time dividing into two sets of multiple cells while traveling down the fallopian tube into the uterus. One set will form the placenta to nourish the fetus, and the other set will become the fetus. The egg will attach to the uterine lining at a gestational age of two weeks.
Weeks Three and Four
The neural tube serves to encase the beginnings of the brain and spinal cord during this period, laying the groundwork for the central nervous system. The heart forms and begins to beat around week three, and the beginnings of the circulatory system appear. The tiny embryo is about as big as an apple seed, and according to the Dr. Spock website is only about one-quarter inch long. The umbilical cord and placenta are now fully functional. By week four, the backbone and muscles begin to develop, as do limbs, eyes and ears, and even hair.
Weeks Five and Six
By the beginning of week five Pregnancy.org states that the embryo is ten thousand times larger than the original fertilized egg, making it the size of a raspberry. During this period, the heart is pumping blood through the circulation system and all four chambers of the heart, while other important physical developments occur. The embryo develops fingers, and although shut, his eyes are darker thanks to the production of pigment. Scientists can detect brain waves at this point, signaling the further development of the brain. Other organs begin to form, including the lungs and the liver, which by the end of week six can take over blood cell production. The umbilical cord has become the embryo's lifeline at this point, and works to provide nutrients, oxygen, and waste disposal.
Weeks Seven and Eight
Brain growth continues during weeks seven and eight, according to the Dr. Spock website, and doctors can see the reproductive organs. The first muscle movements take place during this timeframe. Although not fully developed, by the end of week eight all vital organs are in place. The embryo is ready to begin its next phase as a fetus, and weighs in at about one-third of an ounce.
