Pregnancy Test: What is it?

Pregnancy tests measure the amount of human chorionic gonadotrophin (hCG), the pregnancy hormone, in your body to determine whether you are pregnant. Human chorionic gonadotropin (hCG) is produced by trophoblast cells of the fertilized ovum (blastocyst). HCG, discovered in 1930, cannot be detected until after implantation. If the test is performed too early in the pregnancy, the results will be false negatives. False positives can happen, although they are very rare.

Pregnancy Test:  The History

Pregnancy testing, albeit with more superstitious than scientific results, was part of many ancient civilizations, including ancient Greek and Egyptian cultures. In the ancient Egypt people 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. 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.

The precursor to modern testing based on the presence of human chorionic gonadotropin (hCG) was introduced in 1928. Early studies of hCG had concluded that it was produced by the pituitary gland. In the 1930s, Georgeanna Joness discovered that hCG was produced not by the pituitary gland, but by the placenta. This discovery was important in relying on hCG as an early marker of pregnancy.

Direct measurement of antigens, such as hCG, was made possible with the invention of the radioimmunoassay in 1959. In the 1960s, radioimmunoassays required sophisticated apparatus and special radiation precautions and were expensive. In the 1970s, the discovery of monoclonal antibodies led to the development of the relatively simple and cheap immunoassays, such as agglutination-inhibition-based assays and sandwich ELISA, used in the modern home pregnancy test.

Pregnancy Test:  The Modern Pregnancy Test

Today the quickest test for pregnancy after fertilization 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.

Most chemical tests for pregnancy look for the presence of the beta subunit of hCG or human chorionic gonadotropin in the blood or urine. hCG can be detected in urine or blood after implantation, which occurs six to twelve days after fertilization. Quantitative blood (serum beta) tests are more sensitive than urine tests and can detect hCG levels as low as 1 mIU/mL, although they are expensive and used only in unusual circumstances. The urine test strips have published detection thresholds of 20 mIU/mL to 100 mIU/mL, depending on the brand. Most home pregnancy tests are based on lateral-flow technology.

Pregnancy Test:  When Should You Take One?

The most accurate way to test for pregnancy is to wait until after your period is due. Although some can test earlier, all pregnancy tests will be most accurate if you wait at least until this point.

Some tests may require you to wait longer. This is because the time it takes for the fertilized egg to implant in your uterus wall can vary. A common misconception is that implantation occurs 7 days after conception, but the research findings showed that first appearance of the pregnancy hormone, HCG, occurs 6-12 days after ovulation, with 84% of the pregnancies implanting on days 8-10 after ovulation. If you don’t get a positive pregnancy test, it doesn’t mean that you are not pregnant. It could a false negative – because you ovulated later than you thought (meaning that conception and subsequent HCG production didn’t happen as you calculated) or that, for you, implantation took longer than the average.

Pregnancy Test:  Results of a pregnancy test

• Timing of test. A Pregnancy Test should be done at the right time to ensure that results are correct. The more sensitive tests may be able to detect low levels of hCG as early as four days before your period is due, or seven days after conception. However, most period tests will give accurate results if you test at the time your period would normally be due. If you have a negative result, but still don’t get your period, test again three days later. If you are pregnant, the levels of hormone may build up enough by then to be picked up by the test. The accuracy of a Pregnancy Test is related to the day of ovulation, not of the act of intercourse or insemination that caused the pregnancy. Sperm can live up to five days or longer in the fallopian tubes before they inseminate the egg.
• False negative: A false negative result is where the Pregnancy Test returns negative yet the patient is pregnant. False negative readings can occur when testing is done too early. To avoid false negatives testing should be done after you are due, or which occurs about two weeks after ovulation. By the time that your period is late, the likelihood of a false negative is low. Ovulation tends to occur two weeks before your period, although many women are irregular. Even if you normally have regular menstrual cycles, a number of factors may cause an unexpectedly early or late ovulation. After ovulation it could take up to twelve further days for implantation to occur, although this is rare, meaning even the most sensitive Pregnancy Test may give false negatives up to seventeen days after conception. Because some home pregnancy tests have high hCG detection thresholds (up to 100 mIU/mL), it may take an additional three or four days for hCG to rise to levels detectable by these tests — meaning false negatives may occur up to three weeks after insemination. A false positive result is where the Pregnancy Test outcome indicates positive yet the woman is not pregnant. False positive test are rare (more than false negatives) but they may occur for several reasons. These include: errors of test application, use of drugs containing the assay molecule, and non-pregnant production of the assay molecule. A woman who has been given an hCG injection as part of infertility treatment will test positive on pregnancy tests that assay hCG, regardless of her actual pregnancy status. Some diseases of the liver, cancers, and other medical conditions may produce elevated Human chorionic gonadotropin (hCG) and thus cause a false positive pregnancy test.

Pregnancy Test:  Conclusion

If you had sex without using protection, it is a good idea to take a home pregnancy test. A Pregnancy Test measures HCG, a pregnancy hormone; when it reaches a particular level then the pregnancy results are positive. The best time to take a home Pregnancy Test is to wait until your period is due. If you have negative result, but are worried about false negatives, then wait another three days and retake the test. False positives are rare, but they can happen, so if you think you might have a false positive, you can always retake the test to make sure.

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