Pregnancy test
A pregnancy test is used to determine whether a female is pregnant or not. The two primary methods are testing for the female pregnancy hormone (human chorionic gonadotropin (hCG)) in blood or urine using a pregnancy test kit, and scanning with ultrasonography.[1] Testing blood for hCG results in the earliest detection of pregnancy.[2] Almost all pregnant women will have a positive urine pregnancy test one week after the first day of a missed menstrual period.[3]
Types
Human chorionic gonadotropin (hCG)
Identified in the early 20th century,
Qualitative tests (yes/no or positive/negative results) look for the presence of the
Quantitative tests measure the exact amount of hCG in the sample. Blood tests can detect hCG levels as low as 1 mIU/mL, and typically clinicians will diagnose a positive pregnancy test at 5mIU/mL.[11]
Urine pregnancy test | Blood pregnancy test | |
---|---|---|
Detection thresholds | High-sensitivity:
Qualitative test: 20 to 50 mIU/mL, depending on test Low-sensitivity: Qualitative test: 1500-2000 mIU/mL, depending on test |
Qualitative test:
5 to 10 mIU/mL, depending on test Quantitative test: 1 to 2 mIU/mL for an ultrasensitive test |
There is a multilevel urine pregnancy test (MLPT) that measures hCG levels semiquantitatively. The hCG levels are measured at <25, 25 to 99, 100 to 499, 500 to 1999, 2000 to 9999, and >10,000 mIU/mL. This test has utility for determining the success of medication abortion.[13][14]
Ultrasound
Accuracy
A systematic review published in 1998 showed that home pregnancy test kits, when used by experienced technicians, are almost as accurate as professional laboratory testing (97.4%). When used by consumers, however, the accuracy fell to 75%: the review authors noted that many users misunderstood or failed to follow the instructions included in the kits.[16]
False positive
False positive pregnancy test results are rare and may occur for several reasons, including:
- user error in performing and interpreting the test,
- biochemical pregnancy (loss of pregnancy before signs of pregnancy are apparent on ultrasound, likely very soon after implantation),
- and non-pregnant production of the hCG molecule (i.e. secretion due to a tumor or the pituitary gland, some diseases of the heterophile antibodies, enterocystoplasties, gestational trophoblastic diseases (GTD), and gestational trophoblastic neoplasms).[17][3]
- bacterial contamination and blood in urine[18]
Spurious evaporation lines may appear on many home pregnancy tests if read after the suggested 3–5 minute window or reaction time, independent of an actual pregnancy. False positives may also appear on tests used past their expiration date.[19]
False positive pregnancy test can happen due to 'phantom hCG' which is due to people having human antianimal or heterophilic antibodies.[20]
False positives can also be caused by (in order of incidence) quiescent pregnancy, pituitary sulfated hCG, heterophilic antibody, familial hCG syndrome and cancer.[21]
Due to use of medication
Urine tests can be falsely positive in those that are taking the medications:
False negative
More rare, false negative results can also occur due to a "hook effect", where a sample with a very high level of hCG is tested without dilution, causing an invalid result.[27]
Other uses
Pregnancy tests may be used to predict if a pregnancy is likely to continue or is abnormal. Miscarriage, or spontaneous abortion or pregnancy loss, is common in early pregnancy.[28] Serial quantitative blood tests may be done, usually 48 hours apart, and interpreted based on the knowledge that hCG in a viable normal pregnancy rises rapidly in early pregnancy. For example, for a starting hCG level of 1,500 mIU/ml or less, the hCG of continuing, normal pregnancy will increase at least 49% in 48 hours. However, for pregnancies with a higher starting hCG, between 1,500 and 3,000 mIU/ml, the hCG should rise at least 40%; for a starting hCG greater than 3,000 mIU/ml, the hCG should increase at least 33%.[29] Failure to rise by these minimums may indicate that the pregnancy is not normal, either as a failed intrauterine pregnancy or a possible ectopic pregnancy.[29]
Ultrasound is also a common tool for determining viability and location of a pregnancy. Serial ultrasound may be used to identify non-viable pregnancies, as pregnancies that do not grow in size or develop expected structural findings on repeated ultrasounds over a 1-2 week interval may be identified as abnormal.[30] Occasionally, a single ultrasound may be used to identify a pregnancy as non-viable; for example, an embryo that is greater than a certain size but that lacks a visible heart beat may be confidently determined to be not viable without the need for follow up ultrasound for confirmation.[30]
Research
Research has identified at least one other possible marker that may appear earlier and exclusively during pregnancy. For example, early pregnancy factor (EPF) can be detected in blood within 48 hours of fertilization, rather than after implantation.[31] However, its reliable use as a pregnancy test remains unclear as studies have shown its presence in physiological situations besides pregnancy, and its application to humans remains limited.[32]
History
Records of attempts at pregnancy testing have been found as far back as the ancient Greek and ancient Egyptian cultures. The ancient Egyptians watered bags of wheat and barley with the urine of a possibly pregnant woman. Germination indicated pregnancy. The type of grain that sprouted was taken as an indicator of the fetus's sex.[34] 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.
At the beginning of the 1930s, Hillel Shapiro and Harry Zwarenstein, who were researchers at the University of Cape Town, discovered that if urine from a pregnant woman was injected into the South African Xenopus frog and the frog ovulated, this indicated that the subject was pregnant. This test, known as the frog test, was used throughout the world from the 1930s to 1960s, with Xenopus frogs being exported in great numbers.[37][38] Shapiro's advisor, Lancelot Hogben, claimed to have developed the pregnancy test himself, but refuted by both Shapiro and Zwarenstein in a letter to the British Medical Journal. A later article, independently authored, granted Hogben credit for the principle of using Xenopus to determine gonadotropin levels in a pregnant woman's urine, but not for its usage as a functional pregnancy test.[39]
Hormonal pregnancy tests such as
Immunologic pregnancy tests were introduced in 1960 when Wide and Gemzell presented a test based on in-vitro hemagglutination inhibition. This was a first step away from in-vivo pregnancy testing[41][42] and initiated a series of improvements in pregnancy testing leading to the contemporary at-home testing.[42] Direct measurement of antigens, such as hCG, was made possible after the invention of the radioimmunoassay in 1959.[43] Radioimmunoassays require sophisticated apparatus and special radiation precautions and are expensive.
Another home pregnancy testing kit was based on the work of Judith Vaitukaitis and Glenn Braunstein, who developed a sensitive hCG assay at the National Institutes of Health.[45][46] That test went onto the market in 1978.[47] In the 1970s, the discovery of
See also
References
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- ^ Woo J (2006). "Why and when is Ultrasound used in Pregnancy?". Obstetric Ultrasound: A Comprehensive Guide. Retrieved 27 May 2007.
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- ^ ISBN 978-0-7817-3055-6.
- ^ "First Response early result pregnancy test" (PDF). FirstResponse.com.
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- ^ Clarke FM. Controversies in assisted reproduction and genetics. Does "EPF" have an identity?. J Assist Reprod Genet. 1997;14(9):489–491. doi:10.1023/a:1021110906666
- ^ Clark, Stephanie Brown. (2005).Jan Steen: The Doctor's Visit.Literature, Arts, and Medicine Database. Retrieved 27 May 2007.
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 24 May 2006.
"The Doctor's Visit." (n.d.). The Web Gallery of Art. Retrieved 24 May 2006. - PMID 13960613.
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- ^ Fiala C (29 March 2018). "Museum für Verhütung und Schwangerschaftsabbruch - Museum of Contraception and Abortion". en.muvs.org. Archived from the original on 30 March 2018. Retrieved 29 March 2018.
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- ^ "History of the Pregnancy Test Kit - Home Page". history.nih.gov. Retrieved 4 May 2020.
- ^ A Thin Blue Line: The History of the Pregnancy Test Kit. "A Timeline of Pregnancy Testing". National Institutes of Health. Retrieved 15 March 2015.
- ^ Nudd T (9 January 2018). "Ikea Wants You to Pee on This Ad. If You're Pregnant, It Will Give You a Discount on a Crib". Adweek. Retrieved 13 January 2018.
External links
- Media related to Pregnancy test at Wikimedia Commons