The prenatal period is ten lunar months of twenty-eight days each in length or nine calendar months. However, the period can and does vary greatly in length, ranging from 180-334 days. There are approximately three times as many babies born prematurely as post-maturely. Meredith has reported that the average length of the prenatal period is 38 weeks or 266 days. However, 70% of babies vary from 36 to 40 weeks or 266 days, and 98% range from 34 to 42 weeks (238-294 days). The prenatal period is divided into three stages. These are (i) the period of the zygote (ii) the period of the embryo and (iii) the period of the fetus.
Period of Zygote (fertilization to end of second week)
Half of a person’s genetic material comes from his father and half comes from his mother. These two halves come together to form a unique combination of genetic potentialities when the sperm fertilizes the egg. In the nucleus of the fertilized egg or zygote, are the materials that bear the pattern for a new person, one who is different from his parents and yet like them. This first cell in which the male genes are joined with the female is called the Zygote. The zygote looks like an unfertilized egg. The
egg is so much larger than the sperm that it can absorb the sperm without showing it. However, the unfertilized egg has only twenty-three chromosomes, whereas the zygote has forty-six –the twenty-three that were in the egg originally and the additional twenty-three contributed by the male. Less than two days after the sperm unites with the egg, the zygote divides into two cells. Then these two cells each divide again, and
the process of division goes on, forming in nine months a new human being.
Period of Embryo (end of the second week to end of the second lunar month)
The embryo develops into a miniature human being. This stage begins on the 15th day after conception and continues until about the 8th week, or until the embryo is 1.2 inches in length. During this period the cells of the embryo are not only multiplying, but they are taking on specific functions. This process is called tissue differentiation. It is during this critical period of differentiation (most of the first trimester or three-month period) that the growing fetus is most susceptible to damage from external sources (teratogens) including viral infections such as rubella, x-rays, and other radiation, and poor nutrition.
A child who has one developmental problem may have other problems that arose at the same time. Kidney problems and hearing problems, for example, are often found together because both kidneys and the inner ears develop at the same time. Formation of the heart begins in 3 weeks, the beginning development of the brain and spinal cord, and the beginning of the gastrointestinal tract.
Teratogens introduced during this period may cause severe problems such as the absence of one or more limbs or a heart that is outside of the chest cavity at birth.
Period of Fetus (end of the second lunar month to birth)
At this point, the embryo is developed enough to call a fetus. All organs and structures found in a full-term newborn are present. The head comprises nearly half of the fetus’ size and the face is well formed at weeks 9 to 12 (3 inches, 1 ounce). The eyelids close now and will not reopen until about the 28th week. The tooth buds for the baby teeth appear. The genitalia is now clearly male or female.
Weeks 13 to 16 (6 inches) mark the beginning of the second trimester. Although the skin of the fetus is almost transparent, fine hair develops on the head called lanugo. The fetus makes active movements, including sucking, which leads to some swallowing of the amniotic fluid. A thin dark substance called meconium is made in the intestinal tract. The heart beats 120-150 beats per minute and brain waves are detectable.
Eyebrows and lashes appear and nails appear on fingers and toes at weeks 17 to 20 (8 inches). This is an exciting time for the parents: The mother can feel the fetus moving (quickening) and also hear the heartbeat with the help of a stethoscope.
