Pregnancy test

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A pregnancy test is a test to determine whether or not a woman is pregnant.

Contents 1 Chemical markers 2 Doubling rule 3 Correlation to obstetric ultrasonography 4 False positive/negative tests 5 History 6 References

[edit] Chemical markers

The earliest test for pregnancy is a rosette inhibition assay for early pregnancy factor (EPF). EPF can be detected in blood within 48 hours of fertilization. However, testing for EPF is expensive and time consuming.

All commercially available tests for pregnancy look for the presence of the beta subunit of hCG or human chorionic gonadotropin in the blood or urine. They are usually performed after a missed menstruation or 2-3 weeks after ovulation.

hCG can be detected in urine or blood after implantation, which occurs six to twelve days after fertilization.^[1] Some home pregnancy tests claim to detect hCG as early as 4 days before the next expected period. Whether that statement is accurate for an individual woman depends on the length of her luteal phase and on how soon that individual embryo implanted. Quantitative blood (serum beta) tests can detect hCG levels as low as 1 mIU/mL, while urine tests have published detection thresholds between 20 and 100 mIU/mL, depending on the brand.^[2] Qualitative blood tests generally have a threshold of 25 mIU/mL, and so are less sensitive than some available home pregnancy tests.

This hormone is released by trophoblastic tissue in the placenta. In rare cases, it may be produced by a choriocarcinoma or some other germ cell tumours, or even other forms of cancer, e.g. lung cancer.

[edit] Doubling rule

Doctors can also perform quantitative serum beta hCG levels. The measurement is done in mIU/ml, and a level above 25 mIU/ml usually indicates the presence of a pregnancy. Two tests are usually necessary to verify a normal increase, usually 2-3 days apart. The increase in serum hCG can be interactively monitored. Below an hCG level of 1,200 mIU/ml the hCG usually doubles every 48-72 hours, though a rise of 50-60% is still considered normal. Between 1,200 and 6,000 mIU/ml serum the hCG usually takes 72-96 hours to double, and above 6,000 mIU/ml, the hCG often takes more than four days to double. Failure to increase normally may indicate that the pregnancy is not developing well and can be an early sign for a possible miscarriage or an ectopic pregnancy. After a miscarriage, HCG levels fall steadily back to the non-pregnancy range. Also, an ectopic pregnancy may be suspected when hCG levels fail to double, particularly if the HCG level rises, falls and rises again.

Quantitative beta hCG readings are also used in evaluation of trophoblastic and other germ cell tumors.

[edit] Correlation to obstetric ultrasonography

Once a pregnancy has advanced past the first 6-8 weeks, a pregnancy is usually easier to follow by sonography as more information is obtained in real time. In general a pregnancy is detectable 25 days after ovulation by transvaginal sonography (usually corresponding to an hCG level of > 1,500 miu/ml).

[edit] False positive/negative tests

False negative readings can occur when testing is done too early. Quantitative blood tests and the most sensitive urine tests usually detect hCG shortly after implantation, which can occur anywhere from 6 to 12 days after ovulation.^[1] Beta hCG levels rise exponentially in the first two months or so of pregnancy so the earlier the test is performed, the higher the chance of a false negative result. Less sensitive urine tests and qualitative blood tests may not detect pregnancy until three or four days after implantation. Menstruation occurs on average 14 days after ovulation, so the likelihood of a false negative is low once a menstrual period is late.

False positive results can result from diseases like choriocarcinomas, IgA deficiencies, heterophile antibodies, enterocystoplasties, gestational trophoblastic diseases (GTD), gestational trophoblastic neoplasms (GTN), and testicular germ cell malignancies. Also, many home pregnancy tests show a positive or unclear result when read well after the suggested 3-5 minute window or reaction time, independent of an actual pregnancy; this type of false positive is also known as an evaporation line. Therefore, it is imperative that a pregnancy test is interpreted within the reaction time specified by the test manufacturer. Additionally anyone who has been given an hCG injection may also experience a false positive result. hCG injections are associated with infertility treatments. Common infertility drugs like clomid do not contain the hCG hormone. ^[3]

Some individuals react to some substrate in the test and thus will display a consistently low positive blood pregnancy test even though they are not pregnant. This phantom hCG may lead to serious misdiagnosis and intervention, but can be detected with serial dilutions.^{[citation needed]}

[edit] History

The ancient Egyptians watered bags of wheat and barley with the urine of a possibly pregnant woman. Germination indicated pregnancy, and based on what type of grain sprouted they predicted the gender of the fetus. Hippocrates suggested that a woman who had missed her period should drink a solution of honey in water at bedtime. Resulting abdominal distention and cramps would indicate the presence of a pregnancy. Avicenna and many physicians after him in the Middle Ages performed uroscopy, a nonscientific method to evaluate urine. Selmar Aschheim and Bernhard Zondek introduced testing based on the presence of hCG in 1928. In their test an infantile female mouse was injected subcutaneously with urine of the person to be tested, and some time later the mouse was sacrificed. Presence of ovulation indicated that the urine contained hCG and meant that the person was pregnant. A similar test was developed using immature rabbits, the rabbit test. Here, too, it was necessary to kill the animal to check its ovaries. An improvement arrived with the frog test that was still used in the 1950s. A female frog was injected with serum or urine of the patient. If the frog produced eggs within the next 24 hours, the test was positive. In the frog test, the animal remained alive, and could be used again.

[edit] References

• Speert H: Iconographia Gyniatrica. FA Davis Company, Philadelphia, 1973.
• Cole, L.A. , Khanlian, S.A. Easy fix for clinical laboratories for the false-positive defect with the Abbott AxSym total β-hCG test. USA hCG Reference Service. (2004)

1. ^ ^{a b} Wilcox AJ, Baird DD, Weinberg CR (1999). "Time of implantation of the conceptus and loss of pregnancy.". New England Journal of Medicine 340 (23): 1796-1799. PMID 10362823. 
2. ^ Waddell, Rebecca Smith (2006). FertilityPlus.org. Home Pregnancy Test hCG Levels and FAQ. Retrieved on June 17, 2006.
3. ^ Phillips, Pat (2007). Early Pregnancy Tests. Pregnancy Test FAQ. Retrieved on March 4, 2007.
4. ^ Clark, Stephanie Brown. (2005). Jan Steen: The Doctor's Visit. Literature, Arts, and Medicine Database. Retrieved 2006-05-24.
5. ^ Lubsen-Brandsma, M.A. (1997). Jan Steen's fire pot; pregnancy test or gynecological therapeutic method in the 17th century?. Ned Tijdschr Geneeskd, 141(51), 2513-7. Retrieved 2006-05-24.
6. ^ "The Doctor's Visit." (n.d.). The Web Gallery of Art. Retrieved 2006-05-24.