PREGNANCY

Maternal Development

A woman's physical state begins to change from as early as the implantation of the fertilized egg and continues to change throughout gestation. The ability of a woman to alter herself to support and nurture the development of another being within her own body is one of nature's most impressive feats. From a physiologic standpoint, the maternal body remodels almost all of its organ systems, from heart to hormones, to prepare for the upcoming nine-month gestation. These changes result in the various signs and symptoms characteristic of pregnancy.

In general, a typical gestation, or pregnancy, lasts nine months or three trimesters of three months. Trimesters are used to mark significant milestones in a pregnancy. For example, most spontaneous miscarriages occur prior to the end of the first trimester. The end of the second trimester usually is a good time to recheck maternal lab values, such as the blood count, and to screen for diabetes in pregnancy. From an obstetrician's standpoint, a gestation is measured in weeks. Because different women have different tendencies toward ovulation (some ovulate earlier in their menstrual cycles, some later), it is difficult to establish a gestational age from the time of fertilization. Instead doctors and midwives calculate the gestational age from a more reliable indicator: the first day of the woman's last normal menstrual period. This starting time is usually about two weeks prior to ovulation. The due date can be quickly calculated using a simple formula: adding seven days to the date of the start of the last normal menstrual period, then subtracting three months. The resulting month and day represent the expected delivery date of a full-term gestation.

One of the most obvious signals of pregnancy is the interruption of a woman's menstrual cycle. This sign is most reliable in women who have regular, consistent menses (menstrual flows). A period that is ten days late or more in a woman with regular menses can be considered a strong indicator of pregnancy. This suspicion is strengthened if a woman goes on to skip her next period altogether. This qualification changes for women who have a history of skipping periods or have erratic cycles that are affected by environmental or physical stressors. For these women, a pregnancy test is the best way to ascertain pregnancy.

What is the most reliable way to determine whether a woman is pregnant? There are dozens of home pregnancy tests available. These are good initial measures to use. Although some companies state that their tests are greater than 97 percent accurate, some individuals fail to use these tests properly, which can result in a lower than expected accuracy rate. Studies done in the early 1980s and 1990s showed that the accuracy rates of home pregnancy tests ranged from 70 percent to 83 percent for women who were actually pregnant. The best way to obtain a diagnosis is to undergo a blood test ordered by a doctor and performed by trained technicians. These tests use chemical analysis to measure the presence of a hormone called human chorionic gonadotropin (HCG). HCG is produced by placental cells and is expressed in maternal blood and urine almost immediately from the day the embryo implants in the uterus. These biochemical tests determine the level of hormone in a woman's blood sample. Increasing levels of HCG, along with the other symptoms and signs of pregnancy, provide the most reliable, consistent, and reproducible results for determining pregnancy.

Other symptoms of pregnancy that are commonly seen include nausea, fatigue, changes in urinary habits, and ultimately the perception of fetal movement. Episodes of nausea and occasional vomiting, also known as "morning sickness," occur around six weeks from the start of a woman's last menstrual period. Typically, the woman experiences a few episodes of nausea and vomiting, most commonly for a few hours during the morning. These episodes usually pass by the end of the first trimester. Occasionally, women will have more serious episodes of vomiting marked by increased frequency and intolerance of any food or liquid intake. This condition, known as hyperemesis gravidarum, can persist throughout pregnancy. Treatment entails the use of antinausea medications, and if cases are severe enough, hospitalization for intravenous rehydration.

Changes in urinary habits are noted during the first trimester. At that time, the growing uterus begins to exert more force on the bladder, producing the sensation of fullness and increasing the number of trips to the bathroom. As pregnancy continues, the uterus expands out of the pelvis, relieving some pressure on the bladder and decreasing urinary frequency. As the time of labor approaches, however, the fetus "drops" into the pelvis and reexerts pressure on the bladder, resulting in a return of frequent urination.

The first sensation of fetal movement, also known as the "quickening," is reported by most women to occur between sixteen and twenty weeks. These movements are described as "fluttering" or "tickles" in the abdomen. First-time mothers usually report that the quickening occurs later than women who have previously gone through pregnancy. Although this event is not fully diagnostic of pregnancy by itself, it is a milestone that is noted by many obstetricians and is a good way to roughly judge the gestational age of the pregnancy.

In addition to these self-reported symptoms of pregnancy, an obstetrician can use ultrasonography to definitively identify an early gestation. Using transvaginal ultrasound techniques, an obstetrician can identify a gestational sac as early as two weeks, although four to five weeks is the norm. A yolk sac can be seen as early as three weeks but should be clearly seen by six weeks. At seven weeks, the earliest picture of the developing fetus, known as the fetal pole, can be detected. By eight weeks, the fetal heart can be seen contracting. From this gestational age to about twelve weeks, the size of the fetus, measured from the top of the head to the hips (the crown-rump length), can be compared with the gestational age based on a woman's last menstrual period. These two measures are used to determine the gestational age of the pregnancy and to predict the pregnancy's due date.

As the pregnancy progresses, the uterus continues to enlarge. By twelve weeks of gestation, the uterus becomes perceptible through the abdominal wall. This is usually noticed as a small lump that protrudes from the lower abdomen, slightly above the pelvic bone (pubic symphysis) at the level of the start of pubic hair growth. Starting at twenty weeks, a measurement is regularly taken from the pubis to the top, or fundus, of the uterus during an obstetrical visit. The bladder must be empty to produce an accurate measurement. The resulting measurement in centimeters should roughly equal the number of weeks of pregnancy, with an error of plus or minus two centimeters. This measurement, called the fundal height, may indicate that the fetus is not growing properly (i.e., is too small or too big). If an abnormal result is obtained, an ultrasound can usually be done to check fetal growth and the level of amniotic fluid in the womb. This general principle is applicable to single fetal gestations only, because twins and other multiple pregnancies necessarily produce a larger fundal height.

In addition to these changes in physical stature, the pregnant woman goes through a series of amazing physiologic changes that affect all aspects of the maternal body. From a metabolic perspective, pregnancy necessitates an increased maternal need for nutrients, water, and energy (calories). The fetus is dependent on the expectant mother for all nutritional needs and oxygenation, and it extracts what it needs at the woman's expense. Thus, the woman herself needs to gain weight and increase her caloric consumption to meet her own needs and those of the fetus. The National Research Council's dietary guidelines recommend that pregnant women increase their caloric intake by approximately 300 kilocalories per day. Specifically, a nonpregnant woman requires approximately 2,200 kilocalories per day. A pregnant woman should thus consume 2,500 kilocalories per day.

The demand for iron also increases during pregnancy. The body uses iron to carry oxygen in the blood, which is ultimately transported to the fetus. Thus, it is recommended that women increase their iron intake, especially during the second and third trimesters, when the fetus does the bulk of its growing to reach its physical size. Usually, adequate amounts of iron can be obtained through ingestion of iron rich foods, such as liver, and dark leafy vegetables, such as spinach. Some sources have found, however, that the amount of iron provided by both normal dietary in-take and maternal storage is insufficient to meet pregnancy demands. In fact, the National Academy of Sciences and the American College of Obstetrics and Gynecology recommend that pregnant women receive a supplement of 30 milligrams of iron per day. Most obstetricians recommend that a woman stay on her prenatal vitamin, which should supply enough iron to cover the recommended amount. It is also common practice to check the level of blood (via the hematocrit and hemoglobin tests) both at the start and in the third trimester of pregnancy. If the expectant mother is found to be anemic, she is started on additional iron supplements (ferrous sulfate tablets).

In addition to the increased demand for nutrients, increasing the intake of water is vital to the maintenance of pregnancy. Higher levels of total body water are required to provide the increased fluid volume needed to meet the demands of increased blood flow and circulation to the developing baby. Thus, the pregnant woman's kidney system begins to retain water. Maintaining adequate amounts of fluid intake is also important, as it is easier for pregnant women to become dehydrated, which can lead to pre-term contractions.

The summation of all these dietary and metabolic changes can be seen in the recommendations for weight gain in pregnancy. In a normal nonobese woman, a twenty-five to thirty-five pound weight gain is recommended. This value fluctuates depending on the prepregnancy weight of a woman; specifically, an underweight woman may gain up to forty pounds, while it is recommended that overweight women limit their weight gain to fifteen to twenty pounds. Usually, three to six pounds are gained in the first trimester, with a subsequent gain of one-half to one pound per week thereafter until term. Weight should be measured at every obstetrical visit. If a woman does not show a ten-pound weight gain by the mid-second trimester, her nutritional status should be reviewed. A woman with below average weight gain is at higher risk of producing a low-birthweight and intrauterine growth-restricted infant. Likewise, obese women should be careful about their weight gain as they have an increased risk of producing a large for age (macrosomic) baby, with its associated higher risk of difficult delivery and cesarean delivery.

As each week of the pregnancy passes, the woman will be able to gauge the progression of her pregnancy only through the increasing size of her belly and the amount of activity felt in her uterus. She may not be fully aware of the extent of change occurring with the fetus. Hidden within the woman is a process that is no less fascinating than the changes the woman is undergoing.

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.

See also: BIRTH; CONTRACEPTION; PRENATAL CARE; PRENATAL DEVELOPMENT; REPRODUCTIVE TECHNOLOGIES

Bibliography

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.

Doshi, M. L. "Accuracy of Consumer Performed In-Home Tests for Early Pregnancy Detection." American Journal of Public Health 76 (1986):512-514.

Gabbe, Steven G., Jennifer R. Niebyl, and Joseph L. Simpson, eds. Obstetrics: Normal and Problem Pregnancies. New York: Churchill and Livingstone, 1997.

Jeng, L. L., R. M. Moore, R. G. Kaczmarek, P. J. Placek, and R. A. Bright. "How Frequently Are Home Pregnancy Tests Used? Results from the 1988 National Maternal and Infant Health Survey." Birth 18 (1991):11-13.

National Research Council. Recommended Dietary Allowances, 10th edition. Washington, DC: National Academy Press, 1989.

Valanis, B. G., and C. S. Perlman. "Home Pregnancy Testing Kits:Prevalence of Use, False-Negative Rates, and Compliance with Instructions." American Journal of Public Health 72 (1982):1034-1036.

Garrett Lam