Amenorrhea in a healthy reproductive-aged woman who previously has experienced spontaneous, cyclical, predictable menses is highly suggestive of pregnancy. Menstrual cycles vary appreciably in length among women and even in the same woman (Chap. 5, p. 81). hus, amenorrhea is not a reliable pregnancy indicator until 10 days or more after expected menses have passed. Occasionally, uterine bleeding that mimics menstruation is noted after conception. During the irst month of pregnancy, these episodes are likely the consequence of blastocyst implantation. Still, irst-trimester bleeding should generally prompt evaluation for an abnormal pregnancy.
Of other symptoms, maternal perception of fetal movement depends on factors such as parity and habitus. In general, after a irst successful pregnancy, a woman may irst perceive fetal movements between 16 and 18 weeks' gestation. A primigravida may not appreciate fetal movements until approximately 2 weeks later. At about 20 weeks, depending on maternal habitus, an examiner can begin to detect fetal movements.
Of pregnancy signs, changes in the lower reproductive tract, uterus, and breasts develop early. These are described in detail in Chapter 4 (p. 49).
Detection of hCG in maternal blood and urine is the basis for endocrine assays of pregnancy. Syncytiotrophoblast produces hCG in amounts that increase exponentially during the irst trimester following implantation. A main function of hCG is to prevent involution of the corpus luteum, which is the principal site of progesterone formation during the irst 6 weeks of pregnancy.
With a sensitive test, the hormone can be detected in maternal serum or urine by 8 to 9 days after ovulation. he doubling time of serum hCG concentration is 1.4 to 2.0 days. As shown in Figure 9-2, serum levels range widely and increase from the day of implantation. They reach peak levels at 60 to 70 days. Thereafter, the concentration declines slowly until a plateau is reached at approximately 16 weeks' gestation.
Measurement of hCG his hormone is a glycoprotein with high carbohydrate content. he general structure of hCG is a heterodimer composed of two dissimilar subunits, designated a and �, which are non covalently linked. The a-subunit is identical to those of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and thyroid-stimulating hormone (TSH), but the �-subunit is structurally distinct among these. hus, antibodies were developed with high speciicity for the hCG �-subunit.
FIGURE 9-2 Mean concentration (95% CI) of human chorionic gonadotropin (hCG) in serum of This speciicity allows its detecwomen throughout normal pregnancy. tion, and numerous commercial immunoassays are available for measuring serum and urine hCG levels. Although each immunoassay detects a slightly diferent mixture of hCG variants, its free subunits, or its metabolites, all are appropriate for pregnancy testing (Braunstein, 2014).
Depending on the assay used, the sensitivity for the laboratory detection limit of hCG in serum is 1.0 mIU/mL or even lower (Wilcox, 2001).
False-positive hCG test results are rare (Braunstein, 2002).
erroneously with the test antibody directed to hCG in a given assay. The most common factors are heterophilic antibodies.
hese are produced by an individual and bind to the animal derived test antibodies used in a given immunoassay. Thus, women who have worked closely with animals are more likely to develop these antibodies, and alternative laboratory tech niques are available (American College of Obstetricians and
Gynecologists, 2017a). Elevated hCG levels may also reflect molar pregnancy and its associated cancers (Chap. 20, p. 391).