Pregnancy Diagnosis
INTRODUCTION ¡@

The diagnosis of pregnancy requires a multifaceted approach using 3 main diagnostic tools, history and physical examination, hormonal assays, and ultrasound (US).

Currently, physicians may utilize all of these tools to diagnose pregnancy at early gestation and to help rule out other pathologies.

HISTORY AND PHYSICAL EXAMINATION ¡@

The diagnosis of pregnancy traditionally has been made from history and physical examination. Important aspects of the menstrual history must be obtained. The woman should describe her usual menstrual pattern, including date of onset of last menses, duration, flow, and frequency. Items that may confuse the diagnosis of early pregnancy are an atypical last menstrual period (LMP), contraceptive use, and history of irregular menses. Additionally, as many as 25% of women bleed during their first trimester, further complicating the assessment.

Be alert to the presentation of rising human chorionic gonadotropin (hCG) levels, an empty uterus observed on US, abdominal pain, and vaginal bleeding, as these may signal an ectopic pregnancy. Ectopic pregnancies are the number one cause of first trimester maternal mortality and should be diagnosed early, before the pregnancy ruptures or the patient becomes unstable . Other historical factors related to ectopic pregnancies include prior tubal manipulation, pelvic inflammatory disease (PID), previous ectopic pregnancy, tubal disease, use of an intrauterine device (IUD) for contraception, fertility therapies, and tubal ligation.

The classic presentation of pregnancy is a woman with menses of regular frequency who presents with amenorrhea, nausea, vomiting, generalized malaise, and breast tenderness.

Upon physical examination, one may find an enlarged uterus on bimanual exam, breast changes, and softening and enlargement of the cervix (Hegar sign; observed at approximately 6 wk). The Chadwick sign is a bluish discoloration of the cervix from venous congestion and can be observed by 8-10 weeks. A gravid uterus may be palpable low in the abdomen if the pregnancy has progressed far enough, usually by 12 weeks. Currently, through the use of chemical assays and US, physicians are capable of making the diagnosis of pregnancy before many of the physical signs and symptoms are evident.

HORMONAL ASSAYS ¡@

Several hormones can be measured and monitored to aid in the diagnosis of pregnancy. The most commonly used assay tests are for the beta subunit of hCG. Other hormones that have been utilized include progesterone (P) and early pregnancy factor (EPF).

The cytotrophoblast and syncytiotrophoblast each secrete a variety of hormones that include but are not limited to corticotropin-releasing hormone, gonadotropin-releasing hormone, thyrotropin-releasing hormone, somatostatin, adrenocorticotropic hormone, human chorionic thyrotropin, human placental lactogen, inhibin/activin, transforming growth factor beta, insulinlike growth factors 1 and 2, epidermal growth factor, pregnancy-specific beta-1 glycoprotein, placental protein 5, and pregnancy-associated plasma protein-A. So far, no commercially feasible tests that utilize these hormones have been made available to aid in the diagnosis of pregnancy.

Beta human chorionic gonadotropin

HCG is a glycoprotein similar in structure to follicle-stimulating hormone, luteinizing hormone (LH), and thyroid-stimulating hormone (TSH). The beta subunit of hCG differs from the others in that it has a 30-amino-acid tailpiece at the COOH terminus.

The beta hCG subunit is present in the syncytial layer of the blastomere, and the alpha subunits are localized to the cytotrophoblasts, with none present in the syncytial layer. HCG messenger RNA is detectable in the blastomeres of 6- to 8-cell embryos at 2 days but is not isolated in culture medium until 6 days. Detection in maternal serum and urine is evident only after implantation and vascular communication is established with the decidua by the syncytiotrophoblast 8-10 days after conception. Time of detection is related to the sensitivity of the assay being used. Most current pregnancy tests have a sensitivity of 25 mlU/mL.

Currently, 4 main hCG assays are being used: radioimmunoassay (RIA), immunoradiometric assay (IRMA), enzyme-linked immunosorbent assay (ELISA), and fluoroimmunoassay (FIA). The specifics of each type of test are beyond the scope of this chapter, but characteristics of each are listed as follows:

Dimeric hCG and both the alpha and beta subunits are produced in the pituitary gland of nonpregnant females and released in association with LH. Although levels are much higher in postmenopausal women (110 pg/mL vs 10 pg/mL), they still are below the sensitivity of the most sensitive clinical assays (approximately 1 mlU/mL) used in pregnancy monitoring.

HCG is detectable in the serum of approximately 5% of patients by 8 days after conception and in more than 98% of patients by day 11. At 4 weeks?gestation (18-22 post-conception days), the dimer and beta-subunit hCG doubling times are approximately 2.2 (standard deviation [SD] +/-0.8) days and falls to 3.5 (SD +/-1.2) days by 9 weeks gestation. Levels peak at 10-12 weeks and then begin to decline rapidly until another more gradual rise begins at 22 weeks, which continues until term. The initial rate of rise, measured by serial quantitative hCG levels, is important in the monitoring of early complicated pregnancies that have yet to be documented as viable and intrauterine. Failure to achieve the projected rate of rise may suggest an ectopic pregnancy or spontaneous abortion. On the other hand, an abnormally high level or accelerated rise can prompt investigation into the possibility of molar pregnancy, multiple gestations, or chromosomal abnormalities.

Progesterone

Measuring serum progesterone is a useful test after a positive hCG has been obtained and if a concern exists about ectopic pregnancy or spontaneous abortion. Serum P is a reflection of P production by the corpus luteum, which is stimulated by a viable pregnancy. Measurement of serum P is an inexpensive test and can reliably predict pregnancy prognosis. Currently, RIAs and FIAs are available that can be run in 3-4 hours. A dipstick ELISA test also is marketed that can determine a serum P level of less than 15 ng/mL. The ELISA test is helpful as a screening tool for at-risk populations, because P levels of greater than 15 ng/mL make ectopic pregnancy unlikely.

Viable intrauterine pregnancy can be diagnosed with 97.5% sensitivity if the serum P levels are greater than 25 ng/mL (>79.5 nmol/L). Conversely, serum P levels of less than 5 ng/mL (<15.9 nmol/L) can diagnose a nonviable pregnancy with 100% sensitivity. Measurement of serum P levels of less than 5 ng/mL allows diagnostic evaluation of the uterus in a stable patient, even if an ectopic pregnancy cannot be distinguished from a spontaneous intrauterine abortion beforehand. In the event that the serum P is between 5 and 25 ng/mL, further testing through US, additional hormonal assays, or serial exams is warranted to establish viability of the pregnancy.

Early pregnancy factor

EPF is an assay that may be useful in the future. EPF is a poorly defined immunosuppressive protein that has been isolated in maternal serum shortly after conception and is the earliest available marker to indicate fertilization. It is detectable in the serum 36-48 hours after fertilization, peaks in the early first trimester, and is almost undetectable at term. EPF also appears within 48 hours of successful in vitro fertilization embryo transfers. EPF cannot be detected 24 hours after delivery or at the termination of an ectopic or intrauterine pregnancy. EPF also is undetectable in many ectopic pregnancies and spontaneous abortions, indicating that an inability to identify EPF during pregnancy heralds a poor prognosis.

EPF has limited clinical applications at this time because the molecule is difficult to isolate. Detection of EPF currently relies on a complex and unwieldy assay, the rosette inhibition test. EPF may play a more prominent role in the future as the diagnosis of conception prior to implantation opens doors for contraception, highly accurate dating, and advanced genetic studies.

ULTRASOUND ¡@

With the advent of transvaginal ultrasound (TVUS), the diagnosis of pregnancy can be made even earlier than is capable with transabdominal scans. US has long been used in uncomplicated pregnancies for dating and as a screening exam for fetal anomalies. US typically is not used to diagnose pregnancy unless the patient presents with vaginal bleeding or abdominal pain early in gestation or is a high-risk obstetric patient. TVUS is the most accurate means of confirming intrauterine pregnancy and gestational age during the early first trimester.

TVUS has several advantages over transabdominal ultrasound (TAUS) during early pregnancy. TVUS can detect signs of intrauterine pregnancy approximately 1 week earlier than TAUS. Patients are not required to have a full bladder and are not required to endure uncomfortable pressure on the abdominal wall from the external probe. TVUS also is better when approaching patients who are obese or those who guard during TAUS examination. On the down side, some patients are anxious about the transvaginal probe and may object to its insertion.

Vaginal probes typically are higher frequency (5-8 MHz) than abdominal probes (3-5 MHz). The higher frequency allows for better resolution of the image but less penetration. Also, practice is necessary for familiarization with the orientation on the US monitor when performing TVUS.

The earliest structure identified is the gestational sac (GS). The GS can be seen on TVUS by 4-5 weeks' gestation and grows at a rate of 1 mm/d in early gestation. By 5.5-6 weeks, a double-decidual sign can be seen, which is the GS surrounded by the thickened decidua. The presence of an early GS can be confused with a small collection of fluid or blood or the pseudogestational sac of an ectopic pregnancy. Because of this, the diagnosis of intrauterine pregnancy should not be made with visualization of the GS alone.

The yolk sac can be recognized by 4-5 weeks' gestation and is seen until approximately 10 weeks?gestation. The yolk sac is a small sphere with a hypoechoic center and is located within the GS . Observing a GS that is larger 10 mm without a yolk sac is rare, and if this is observed, it most likely represents an abnormal pregnancy . Likewise, a yolk sac larger than 7 mm without evidence of a developing fetal pole suggests a nonviable pregnancy. The diagnosis of intrauterine pregnancy can be made once the yolk sac is present, which also rules out an ectopic pregnancy, except in the rare instance of heterotopic pregnancy. Heterotopic pregnancy, an intrauterine pregnancy, and an ectopic pregnancy during the same gestation was once thought to be extremely rare but now has been shown to be present in as many as 1 per 3000 pregnancies.

The fetal or embryonic pole is first seen on TVUS at approximately 5-6 weeks?gestation. The fetal pole is a linear hyperechoic structure that grows at approximately 1 mm/d. Cardiac motion sometimes can be identified in a 2- to 3-mm embryo but almost always is present when the embryo grows to 5 mm or longer.

ULTRASOUND AND HUMAN CHORIONIC GONADOTROPIN ¡@

US becomes even more useful for the diagnosis of early pregnancy and for identifying abnormal pregnancies when it is used in conjunction with quantitative hCG levels. The identification of gestational structures by US correlates with specific levels of hCG, called discriminatory levels. A discriminatory level is the level of hCG at which the structure in question should always be identified. The GS has been identified by TVUS with hCG levels as low as 300 mIU/mL, and the majority of experienced TVUS users should visualize the GS when levels are greater than 2000 mIU/mL. The discriminatory level for the GS is approximately 3600 mIU/mL, and if it is not seen at this point, other pathology must be ruled out. Many use a more conservative discriminatory level for the GS at 2000 mIU/ml, and will begin to rule out pathology if the GS is not seen by TVUS. Furthermore, one study showed that all viable intrauterine pregnancies had a GS identified by TAUS for hCG levels of greater than 6500 mIU/mL.

The usefulness of TVUS evaluation when the hCG is less than 1000 mIU/mL has been debated. One study showed that valuable information still can be garnered in women presenting for emergent TVUS examination with an hCG of less than 1000 mIU/mL. In this study, approximately 13% of the abnormal intrauterine pregnancies and 39% of the ectopic pregnancies were identified by TVUS. US studies should not be delayed purely on the basis of hCG levels.

Other structures also are anticipated in correlation with specific hCG levels. The yolk sac commonly is observed with an hCG level of approximately 2500 mIU/mL, although it may not be identified until levels are much higher. The embryonic pole usually becomes evident at a level of approximately 5000 mIU/mL, and the fetal heartbeat can be seen in the vast majority of normal gestations when the hCG level reaches 10,000 mIU/mL.

CONCLUSION ¡@

The diagnosis of pregnancy can be made by several methods. Normocyclic women who present with amenorrhea and typical history and physical exam findings have the classic presentation and can be diagnosed with a viable intrauterine pregnancy if they progress appropriately. Currently, most women are diagnosed with pregnancy after a missed menstrual cycle and a positive urine or serum hCG. The pregnancy is diagnosed as viable with serial exams and normal pregnancy development, a normal dating ultrasound, or positive fetal heart tones by Doppler.

Women who are high-risk or present with abdominal pain or vaginal bleeding in early gestation are more likely to be evaluated by US and additional hormonal assays. A number of different combinations can diagnose a viable intrauterine pregnancy. The physician must ascertain what is most appropriate at the time of patient presentation.

BIBLIOGRAPHY ¡@