Germinal Period
The period of human development before birth is known as the prenatal stage and is divided into three sections---the germinal period, the embryonic period and the fetal period. During the germinal period, which begins with fertilization and lasts 2 weeks, cells within the fertilized egg begin to divide and multiply, forming a ball of cells known as a blastocyst. This blastocyst connects to the uterine wall, developing a placenta for nutrients and oxygen while releasing hormones to stop menstruation and prevent the mother's immune system from attacking.
Embryonic Period
The embryonic period, which lasts from the middle of the second week until the eighth week, promotes the physical development of body parts. At this point, the cells clearly look human; the heart, lungs and other vital organs form along with fingers, toenails and eyes. This is considered the most critical part of pregnancy because teratogens---free-floating germs and toxins from the mother---can affect and change fetal development.
Fetal Period
Beginning at the end of 9 weeks and ending with birth, the fetal period continues the development and differentiation of structures lied down during the embryonic period. During the third and fourth months, the fetus grows rapidly five to six times larger, from 1 inch to 5 to 6 inches. The origins of learning behavior begin around 16 weeks into pregnancy.
Principles that Guide Development
Fetal development expands from head to foot, in what scientists call a cephalocaudal direction, meaning structures around the head begin first before lower body parts. Additionally, development starts more basic and then switches to more specialized forms, where the building block of a hand appears before fingers, joints and toes show up in later development. Also, more important structures like the heart and brain develop before less important ones like hair and nails.
The Final Months of Gestation
From 26 weeks to 40 weeks after fertilization, known as the third trimester, the fetus rapidly increases in body fat composition. Bones, now fully formed, are soft and flexible, even as calcium, phosphorous and iron nutrients become more available. At 26 weeks, the fetus has a 50 percent chance of survival or better with birth usually occurring between the 35th and 40th weeks. According to research, the fetus does not experience pain until the 28th week, which is around the seventh month of pregnancy. Scientists make these conclusions based on evidence of brain development, insisting that early movements are more likely due to reflexes than actual responses to stimuli.
