Anatomy and Physiology of Pregnancy

Anatomy and Physiology of Pregnancy

Deitra Leonard Lowdermilk

Key Terms and Definitions

Gravidity and Parity image

An understanding of the terms that are used to describe pregnancy and the pregnant woman is essential to the study of maternity care. Box 6-1 describes these terms.

BOX 6-1

Definitions Related to Gravidity and Parity

• Gravida: a woman who is pregnant

• Gravidity: pregnancy

• Multigravida: a woman who has had two or more pregnancies

• Multipara: a woman who has completed two or more pregnancies to 20 weeks of gestation or more

• Nulligravida: a woman who has never been pregnant

• Nullipara: a woman who has not completed a pregnancy with a fetus or fetuses beyond 20 weeks of gestation

• Parity: the number of pregnancies in which the fetus or fetuses have reached 20 weeks of gestation when they are born, not the number of fetuses (e.g., twins) born. Whether the fetus is born alive or is stillborn (fetus who shows no signs of life at birth) does not affect parity.

• Postdate or postterm: a pregnancy that goes beyond 42 weeks of gestation

• Preterm: a pregnancy that has reached 20 weeks of gestation but before completion of 37 weeks of gestation

• Primigravida: a woman who is pregnant for the first time

• Primipara: a woman who has completed one pregnancy with a fetus or fetuses who have reached 20 weeks of gestation

• Term: a pregnancy from the completion of week 37 of gestation to the end of week 42 of gestation

• Viability: capacity to live outside the uterus; there are no clear limits of gestational age or weight but it is rare for a fetus to survive before 22 to 24 weeks of gestation and weighing less than 500 grams

Sources: Cunningham, F., Leveno, K., Bloom, S., Hauth, J., Gilstrap, L., & Wenstrom, K. (2005). Williams obstetrics (22nd ed.). New York: McGraw-Hill; Katz, V. (2007). Spontaneous and recurrent abortions. In V. Katz, G. Lentz, R. Lobo, & D. Gershenson (Eds.). Comprehensive gynecology (5th ed.). Philadelphia: Mosby.

Gravidity and parity information is obtained during history-taking interviews. Obtaining and documenting this information accurately is important in making a plan of care for the pregnant woman.

Two commonly used systems of summarizing the obstetrical history are discussed here. Gravidity and parity may be described with only two digits: The first digit represents the number of pregnancies the woman has had, including the present one; and parity is the number of pregnancies that have reached 20 or more weeks of gestation. For example, if the woman had twins at 36 weeks with her first pregnancy, parity would still be counted as one birth (gravida [G] 1, para [P]1) (Cunningham, Leveno, Bloom, Hauth, Gilstrap, & Wenstrom, 2005). If she becomes pregnant a second time, she would be G2 P1 until she gives birth at 38 weeks when she would then become G2 P2. Another system, which consists of five digits separated with hyphens, is commonly used in maternity centers. This system provides more information about the woman’s obstetric history, although it may not provide accurate information about parity since it provides information about births and not pregnancies reaching 20 weeks of gestation (Beebe, 2005). The first digit represents gravidity, the second digit represents the total number of term births, the third indicates the number of preterm births, the fourth identifies the number of abortions (miscarriage or elective termination of pregnancy), and the fifth is the number of children currently living. The acronym GTPAL (gravidity, term, preterm, abortions, living children) may be helpful in remembering this system of notation. For example, if a woman pregnant only once gives birth at week 34 and the infant survives, the abbreviation that represents this information is 1-0-1-0-1. During her next pregnancy, the abbreviation is 2-0-1-0-1. Additional examples are given in Table 6-1.

Pregnancy Tests image

Early detection of pregnancy allows early initiation of care. Human chorionic gonadotropin (hCG) is the earliest biochemical marker for pregnancy, and pregnancy tests are based on the recognition of hCG or a beta (β) subunit of hCG. Production of β-hCG begins as early as the day of implantation and can be detected as early as 7 to 10 days after conception (Blackburn, 2007). The level of hCG increases until it peaks at approximately 60 to 70 days of gestation and then declines until about 80 days of pregnancy. It remains stable until approximately 30 weeks and then gradually increases until term. Higher than normal levels of hCG may indicate abnormal gestation (e.g., fetus with Down syndrome), or multiple gestation; an abnormally slow increase or a decrease in hCG levels may indicate impending miscarriage (Cunningham et al., 2005).

Serum and urine pregnancy tests are performed in clinics, offices, women’s health centers, and laboratory settings, and urine pregnancy tests may be performed at home. Both serum and urine tests can provide accurate results. A 7- to 10-ml sample of venous blood is collected for serum testing. Most urine tests require a first-voided morning urine specimen because it contains levels of hCG approximately the same as those in serum. Random urine samples usually have lower levels. Urine tests are less expensive and provide more immediate results than do serum tests.

Many different pregnancy tests are available (Fig. 6-1). The wide variety of tests precludes discussion of each. The nurse should read the manufacturer’s directions for the test that is to be used.

Enzyme-linked immunosorbent assay (ELISA) testing is the most popular method of testing for pregnancy. It uses a specific monoclonal antibody (anti-hCG) with enzymes to bond with hCG in urine. ELISA technology is the basis for most over-the-counter home pregnancy tests. With these one-step tests the woman usually applies urine to a strip or absorbent tipped applicator and reads the results. The test kits come with directions for collection of the specimen, the testing procedure, and reading of the results. A positive test result is indicated by a simple color change reaction or a digital reading. Most manufacturers of the kits provide a toll-free telephone number to call if users have concerns and questions about test procedures or results (see Teaching Guidelines). The most common error in performing home pregnancy tests is performing the test too early in pregnancy (Pagana & Pagana, 2006).

Interpreting the results of pregnancy tests requires some judgment. The type of pregnancy test and its degree of sensitivity (ability to detect low levels of a substance) and specificity (ability to discern the absence of a substance) must be considered in conjunction with the woman’s history. This history includes the date of her last normal menstrual period (LNMP), her usual cycle length, and results of previous pregnancy tests. Knowing if the woman is a substance abuser and what medications she is taking is important because medications such as anticonvulsants and tranquilizers can cause false-positive results, whereas diuretics and promethazine can cause false-negative results (Pagana & Pagana, 2006). Improper collection of the specimen, hormone-producing tumors, and laboratory errors also may cause false results.

Depending on the specific test, levels of hCG as low as 6.3 milli-international units/ml can be detected as early as the first day of a missed menstrual period as reported by Cole, Sutton-Riley, Khanlian, Borkovskaya, Rayburn, & Rayburn (2005). These researchers found that most of the over-the-counter pregnancy tests in the study were less sensitive (25 to 100 milli-international units/ml) and detected only a small percentage of pregnancies on the first day of a missed period even though most products claimed to be 99% accurate. Tomlinson, Marshall, and Ellis (2008) found that digital readings of low hCG levels (i.e., 25 milli-international units/ml) were more accurately interpreted by consumers than nondigital tests.

Women who use a home pregnancy test should be advised about the variations in accuracy reporting and to use caution when interpreting results. Whenever any question arises, further evaluation or retesting is appropriate.

Adaptations to Pregnancy image

Maternal physiologic adaptations are attributed to the hormones of pregnancy and to mechanical pressures arising from the enlarging uterus and other tissues. These adaptations protect the woman’s normal physiologic functioning, meet the metabolic demands pregnancy imposes on her body, and provide a nurturing environment for fetal development and growth. Although pregnancy is a normal phenomenon, problems can occur.

Signs of Pregnancy

Some of the physiologic adaptations are recognized as signs and symptoms of pregnancy. Three commonly used categories of signs and symptoms of pregnancy are presumptive (specific changes felt by the woman—e.g., amenorrhea, fatigue, nausea and vomiting, breast changes), probable (changes observed by an examiner—e.g., Hegar sign, ballottement, pregnancy tests), and positive (signs that are attributable only to the presence of the fetus—e.g., hearing fetal heart tones, visualization of the fetus, palpating fetal movements). Table 6-2 summarizes these signs of pregnancy in relation to when they might occur and other causes for their occurrence.

Reproductive System and Breasts


Changes in size, shape, and position.

The phenomenal uterine growth in the first trimester is stimulated by high levels of estrogen and progesterone. Early uterine enlargement results from increased vascularity and dilation of blood vessels, hyperplasia (production of new muscle fibers and fibroelastic tissue) and hypertrophy (enlargement of preexisting muscle fibers and fibroelastic tissue), and development of the decidua. By 7 weeks of gestation the uterus is the size of a large hen’s egg, by 10 weeks of gestation, it is the size of an orange (twice its nonpregnant size), and by 12 weeks of gestation, it is the size of a grapefruit. After the third month, the continuing uterine enlargement is primarily the result of mechanical pressure of the growing fetus.

As the uterus enlarges, it also changes in shape and position. At conception the uterus is shaped like an upside-down pear. During the second trimester, as the muscular walls strengthen and become more elastic, the uterus becomes spherical or globular. Later, as the fetus lengthens, the uterus becomes larger and more ovoid and rises out of the pelvis into the abdominal cavity.

The pregnancy may “show” after the fourteenth week, although this depends to some degree on the woman’s height and weight. Abdominal enlargement may be less apparent in the nullipara with good abdominal muscle tone (Fig. 6-2). Posture also influences the type and degree of abdominal enlargement that occurs. In normal pregnancies, the uterus enlarges at a predictable rate. As the uterus grows, it may be palpated above the symphysis pubis some time between the twelfth and fourteenth weeks of pregnancy (Fig. 6-3). The uterus rises gradually to the level of the umbilicus at about 22 weeks of gestation and nearly reaches the xiphoid process at term. Between weeks 38 and 40, fundal height drops as the fetus begins to descend and engage in the pelvis (lightening) (see Fig. 6-3, dashed line). Generally, lightening occurs in the nullipara approximately 2 weeks before the onset of labor and at the start of labor in the multipara.

Uterine enlargement is determined by measuring fundal height, a measurement commonly used to estimate the duration of pregnancy. However, variation in the position of the fundus or the fetus, variations in the amount of amniotic fluid present, the presence of more than one fetus, maternal obesity, and variation in examiner techniques can reduce the accuracy of this estimation of the duration of pregnancy.

The uterus normally rotates to the right as it elevates, probably because of the presence of the rectosigmoid colon on the left side, but the extensive hypertrophy (enlargement) of the round ligaments keeps the uterus in the midline. Eventually the growing uterus touches the anterior abdominal wall and displaces the intestines to either side of the abdomen (Fig. 6-4). Whenever a pregnant woman is standing, most of her uterus rests against the anterior abdominal wall, and this contributes to altering her center of gravity.

At approximately 6 weeks of gestation, softening and compressibility of the lower uterine segment (the uterine isthmus) occur (Hegar sign) (Fig. 6-5). This change results in exaggerated uterine anteflexion during the first 3 months of pregnancy. In this position, the uterine fundus presses on the urinary bladder, causing the woman to have urinary frequency.

Uteroplacental blood flow.

Placental perfusion depends on the maternal blood flow to the uterus. Blood flow increases rapidly as the uterus increases in size. Although uterine blood flow increases 20-fold the fetoplacental unit grows more rapidly. Consequently, more oxygen is extracted from the uterine blood during the latter part of pregnancy (Cunningham et al., 2005). In a normal term pregnancy, one sixth of the total maternal blood volume is within the uterine vascular system. The rate of blood flow through the uterus averages 500 ml/min, and oxygen consumption of the gravid uterus increases to meet fetal needs. A low maternal arterial pressure, contractions of the uterus, and maternal supine position are three factors known to decrease blood flow. Estrogen stimulation may increase uterine blood flow. Doppler ultrasound examination can be used to measure uterine blood flow velocity, especially in pregnancies at risk because of conditions associated with decreased placental perfusion such as hypertension, intrauterine growth restriction, diabetes mellitus, and multiple gestation (Blackburn, 2007). By using an ultrasound device or a fetal stethoscope, the health care provider may hear the uterine souffle (sound made by blood in the uterine arteries that is synchronous with the maternal pulse) or the funic souffle (sound made by blood rushing through the umbilical vessels and synchronous with the fetal heart rate).

Cervical changes.

A softening of the cervical tip called Goodell sign may be observed at approximately the beginning of the sixth week in a normal, unscarred cervix. This sign is brought about by increased vascularity, slight hypertrophy, and hyperplasia (increase in the number of cells) of the muscle and its collagen-rich connective tissue, which becomes loose, edematous, highly elastic, and increased in volume. The glands near the external os proliferate beneath the stratified squamous epithelium, giving the cervix the velvety appearance characteristic of pregnancy. Friability is increased and may cause slight bleeding after coitus with deep penetration or after vaginal examination. Pregnancy also can cause the squamocolumnar junction, the site for obtaining cells for cervical cancer screening, to be located away from the cervix. Because of all these changes, evaluation of abnormal Papanicolaou (Pap) tests during pregnancy can be complicated. A careful assessment of all pregnant women is important, however, because approximately 3% of all cervical cancers are diagnosed during pregnancy (Copeland & Landon, 2007).

The cervix of the nullipara is rounded. Lacerations of the cervix almost always occur during the birth process. With or without lacerations, however, after childbirth, the cervix becomes more oval in the horizontal plane, and the external os appears as a transverse slit (see Fig. 6-2).

Changes related to the presence of the fetus.

Passive movement of the unengaged fetus is called ballottement and can be identified generally between the sixteenth and eighteenth week. Ballottement is a technique of palpating a floating structure by bouncing it gently and feeling it rebound. In the technique used to palpate the fetus, the examiner places a finger in the vagina and taps gently upward, causing the fetus to rise. The fetus then sinks, and a gentle tap is felt on the finger (Fig. 6-6).

Oct 8, 2016 | Posted by in NURSING | Comments Off on Anatomy and Physiology of Pregnancy
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