Diagnosis of Pregnancy

DIAGNOSIS OF PREGNANCY

For a woman with regular menstrual cycles, a history of one or more missed periods following a time of sexual activity without effective contraception strongly suggests early pregnancy. Fatigue, nausea/vomiting, and breast tenderness are often associated symptoms.

On physical examination, softening and enlargement of the pregnant uterus becomes apparent 6 or more weeks after the last normal menstrual period. At approximately 12 weeks of gestation (12 weeks from the onset of the last menstrual period), the uterus is generally enlarged suffi-ciently to be palpable in the lower abdomen. Other genital tract findings early in pregnancy include congestion and a bluish discoloration of the vagina (Chadwick sign) and softening of the cervix (Hegar sign). Increased pigmenta-tion of the skin and the appearance of circumlinear striae on the abdominal wall occur later in pregnancy and are associ-ated with progesterone effects and physical stretching of the dermis. Palpation of fetal parts and the appreciation of fetal movement and fetal heart tones are diagnostic of preg-nancy, but at a more advanced gestational age. The patient’s initial perception of fetal movement (called “quickening”) is not usually reported before 16 to 18 weeks of gestation, and often as late as 20 weeks in first-time mothers.

Pregnancy cannot be diagnosed only on the basis of symptoms and subjective physical findings. A pregnancytest is needed to confirm the diagnosis. Once a positivepregnancy test is identified and before fetal heart activity (beating fetal heart) is seen on ultrasound, the physician and patient must be aware of signs and symptoms of an abnor-mal pregnancy, including those associated with sponta-neous abortion, ectopic pregnancy, and trophoblastic disease. Several types of urine pregnancy tests are available that measure human chorionic gonadotropin (hCG) produced in the syncytiotrophoblast of the growing pla-centa. Because hCG shares an α-subunit with luteinizing hormone (LH), interpretation of any test that does not differentiate LH from hCG must take into account this overlap in structure. The concentration of hCG necessary to evoke a positive test result must therefore be high enough to avoid a false-positive diagnosis of pregnancy. Standard laboratory urine pregnancy tests become positive approximately 4 weeks following the first day of the last menstrual period (i.e., around the time of the missed period). Home urine pregnancy tests have a low false-positive rate but a high false-negative rate (the test result is negative even though the patient is pregnant). All urine pregnancy tests arebest performed on early-morning urine specimens, which con-tain the highest concentration of hCG Serum pregnancy tests are more specific and sensi-tive because they test for the unique β-subunit of hCG, allowing detection of pregnancy very early in gestation, often before the patient has missed a period. During the first few weeks, the status of a pregnancy may be evaluated by following serial quantitative hCG levels and compar-ing them to the expected rise derived from normative data for proven normal intrauterine pregnancies. Such serial studies often allow differentiation of normal and abnormal pregnancy, or indicate that further testing of other kinds is needed for the same purpose.

Ultrasound examination can detect pregnancy earlyin gestation. With an abdominal ultrasound, the ultrasound transducer is placed on the maternal abdomen, allowing visualization of a normal pregnancy gestational sac 5 to 6 weeks after the beginning of the last normal menstrual period (corresponding to β-hCG concentrations of 5000 to 6000 mIU/mL) Transvaginal ultrasound often detects pregnancy at 3 to 4 weeks of gestation (corresponding to β-hCG concentrations of 1000 to 2000 mIU/mL) becausethe probe is placed in the posterior fornix of the vagina only a few centimeters from the uterine cavity, compared to the relatively longer distance from the abdominal wall to the same location. If the β-hCG concentration is >4000 mIU/ mL, the embryo should be visualized and cardiac activity detected by all ultrasound techniques.

Detection of fetal heart activity (“fetal heart tones”) is also almost always evidence of a viable intrauterine preg-nancy. With a traditional, nonelectronic, acoustic fetoscope, auscultation of fetal heart tones is possible at or beyond to 20 weeks of gestational age. The commonly used elec-tronic Doppler devices can detect fetal heart tones at approximately 12 weeks of gestation.