Pregnancy - Fetal Development
The first trimester is the most critical period for fetal development. It is at this point that the fertilized egg begins to develop from a mass of disparate cells into an organized whole that is truly the sum of its individual specialized parts. The start of the third week after fertilization marks the beginning of the embryonic period. At this point, the fertilized egg begins to differentiate its cells into the beginnings of the placenta and the body of the future fetus. By the end of the fourth week after ovulation, the embryo is roughly four to five millimeters (0.16 to 0.2 inches) long, and heart activity can be seen on ultrasound. By the end of the sixth week, the embryo is roughly two centimeters (three-quarters of an inch) long and has a definitive head separate from the body. A vast amount of organ development occurs before many women realize that they are indeed pregnant, highlighting the importance of attention to the health of women as they anticipate pregnancy.
The fetal period is usually considered to start by the eighth week after fertilization. By this time, the period most crucial to organ and structural development has passed. Development from this period consists of the growth and maturation of structures that were formed during the embryonic period. According to Williams Obstetrics, a primary textbook in the obstetrical field, the milestones in fetal development can be marked every four weeks of the fetus's menstrual/gestational age.
At twelve weeks from the last menstrual period, the fetus is clearly visible by transvaginal ultrasound and may be visualized by abdominal technique depending on the quality of the equipment and the size of the expectant mother. Fingers and toes are differentiated from each other, and fingernails are present. The external genitalia are starting to develop, but it is difficult to determine gender at this point.
By sixteen weeks, the fetus can be seen by abdominal ultrasound. The fetus now weighs approximately 110 grams (four ounces) and has well-developed lower limbs. Intestines, stomach, and bladder should be visible. The sex of the baby can be reliably determined at this point, with fetal cooperation, of course. At this time, most pregnant women undergo screening tests of their blood to check for possible problems with Down syndrome or spina bifida. An ultrasound to assess anatomy and to look for any signs of structural defects is usually done at this age.
The end of the twentieth week represents the midpoint of pregnancy. The average fetus weighs approximately 320 grams (eleven ounces) and is approximately sixteen centimeters (six and one-quarter inches) long. At this point, some scalp hair may be seen in ultrasound images, and the body is covered with a fine, downy hair called lanugo.
The twenty-fourth week represents a major milestone as hospitals with high-tech (level 3) neonatal intensive care nurseries and neonatal specialists consider fetuses at this age to be viable. Unfortunately, fetuses born at this age are extremely premature. The skin is wrinkled, with small amounts of subcutaneous (below the skin) fat present; thus, they have tremendous problems maintaining body temperature. They weigh only 630 grams (twenty-two ounces) and have poorly developed lungs, which necessitates the use of ventilators to assist breathing.
At twenty-eight weeks, the fetus is now roughly twenty-five centimeters (nine and three-quarter inches) long and can weigh approximately 1,100 grams (two pounds, six ounces). By this point, the fetus that is delivered will have eyes partially open, limbs that can move energetically, and may be able to cry. Most fetuses born at this point will survive under the care of a high-tech, level 3 neonatal intensive care unit.
By thirty-two weeks, the average fetus weighs roughly 1,800 grams (three pounds, fifteen ounces), and measures approximately 28 centimeters. The skin is wrinkled and red, but the body begins to fill out with more deposition of subcutaneous fat. This represents another major milestone as the chances of the other problems of prematurity, such as hemorrhages in the brain or the eye and problems with the intestines, drop considerably. The biggest problems facing babies born at this age involve lung development and function.
The thirty-sixth week represents another important milestone. At this point, the baby measures approximately thirty-two centimeters (twelve and one-half inches) and weighs approximately 2,500 grams (five pounds, eight ounces). The body is filled out with subcutaneous fat. Although babies born at this age are still technically "preterm" (infants are considered term at thirty-seven to forty weeks), most women who go into labor at this age would not be stopped with medications. Babies born at this age have an excellent chance of survival.
Finally, at forty weeks, the goal of gestation is reached. At this age, the average baby measures thirty-six centimeters (fourteen inches) and weighs approximately 3,400 grams (seven pounds, seven ounces). The skin is smooth and pink, the body is plump, and the lungs generally function well. It should be noted that these stages of development represent general characteristics only. There is always a large degree of variability in fetal development, much of which is influenced by variable factors such as genetic makeup (e.g., chromosomal defects), characteristics of the parents (e.g., size, weight, race) and characteristics of the pregnancy (e.g., toxemia, maternal smoking, bleeding during the pregnancy). At times, the gestation may go past forty weeks. This occurrence is not uncommon. In fact, very few deliveries occur at exactly forty weeks. In cases where pregnancies go "post-term," the women should be followed normally. Most obstetricians will institute measures to induce delivery if the woman does not go into labor on her own past forty-two weeks. At this point, there is concern that the fetus will grow too big and an increased chance of cesarean delivery. Furthermore, the placenta has a finite "lifespan," and gestations that continue too long past the due date have a higher risk for placental failure, which could harm the safety of the fetus.
Although the gestation itself represents a relatively short period in a woman's life, the physical and personal changes that pregnancy brings about in a woman last throughout her lifetime. Clearly, pregnancy is a task that should not be met without preparation and assistance.
Cunningham, F. Gary, Paul C. MacDonald, Norman F. Gant, Kenneth J. Leveno, Larry C. Gilstrap, Gary D. Hankins, and Steven L. Clark, eds. Williams Obstetrics. Stamford, CT: Simon and Schuster, 1997.
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Gabbe, Steven G., Jennifer R. Niebyl, and Joseph L. Simpson, eds. Obstetrics: Normal and Problem Pregnancies. New York: Churchill and Livingstone, 1997.
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