It is advantageous to diagnose pregnancy as promptly as possible when a sexually active woman misses a menstrual period or has symptoms suggestive of pregnancy. In the event of a desired pregnancy, prenatal care can begin early, and potentially harmful medications and activities such as drug and alcohol use, smoking, and occupational chemical exposure can be halted. In the event of an unwanted pregnancy, counseling about adoption or termination of the pregnancy can be provided at an early stage.
All urine or blood pregnancy tests rely on the detection of human chorionic gonadotropin (hCG) produced by the placenta. Levels increase shortly after implantation, approximately double every 48 hours (this rise can range from 30% to 100% in normal pregnancies), reach a peak at 50–75 days, and fall to lower levels in the second and third trimesters. Pregnancy tests are performed on serum or urine and are accurate at the time of the missed period or shortly after it.
Compared with intrauterine pregnancies, ectopic pregnancies may show lower levels of hCG that plateau or fall in serial determinations. Quantitative assays of hCG repeated at 48-hour intervals are used in the diagnosis of ectopic pregnancy as well as in cases of molar pregnancy, threatened abortion, and missed abortion. Comparison of hCG levels between laboratories may be misleading in a given patient because different international standards may produce results that vary by as much as twofold. hCG levels can also be problematic because they require a series of measurements. Progesterone levels, however, remain relatively stable in the first trimester. A single measurement of progesterone is the best indicator of whether a pregnancy is viable, although there is a broad indeterminate zone. A value less than 5 ng/mL (16 nmol/L) predicts pregnancy failure while a value greater than 25 ng/mL (80 nmoL/L) indicates a pregnancy will be successful. There is uncertainty when the value is between these two points. Combining several serum biomarkers (beta hCG and progesterone) may provide a better prediction of pregnancy viability. Pregnancy of unknown location is a term used to describe a situation where a woman has a positive pregnancy test but the location and viability of the pregnancy is not known because it is not seen on transvaginal ultrasound.
et al. Diagnosing ectopic pregnancy and current concepts in the management of pregnancy of unknown location. Hum Reprod Update. 2014 March–April;20(2):250–61.
et al. Biomarkers for ectopic pregnancy and pregnancy of unknown location. Fertil Steril. 2013 Mar 15;99(4):1107–16.
Manifestations of Pregnancy
The following symptoms and signs are usually due to pregnancy, but none are diagnostic. A record of the time and frequency of coitus is helpful for diagnosing and dating a pregnancy.