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Pregnancy test

A pregnancy test is done to determine whether or not a woman is pregnant. All tests currently available to depend on testing for the presence of beta HCG or human chorionic gonadotropin in the blood or urine.

Pregnancy tests measure the levels of hCG in the blood or urine to indicate the presence or absence of a fertilized egg. In particular, most pregnancy tests employ an antibody that is specific to the β-subunit of hCG (βhCG). This is important so that tests do not make false positives by confusing hCG with LH and FSH. (The latter two are always present at varying levels in the body, while hCG levels are negligible except during pregnancy.)

This hormone is only released by trophoblastic tissue, which usually is produced by a growing fetus and its associated placenta or, rarely, from a choriocarcinoma or some other germ cell tumours.

False negative readings (from home kits) can result from the sensitivity of the test being lower than the current concentration of the HCG in the woman's urine. This can occur quite commonly before the 6th week of pregnancy (defined as 6 weeks from the last menstrual period) - 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.

False positive results can result from rare beta-HCG producing tumors like choriocarcinomas.

Reference levels

A single HCG level cannot indicate a normal pregnancy. In a normal pregnancy there is a regular increase of the HCG, so you need to see more than one value before you can better understand what's going on. Additionally, a single value cannot always tell you how far along the pregnancy is, because of the wide range of blood HCG levels in pregnancy.

Sample reference list for HCG levels in singleton pregnancies, based on days past ovulation (DPO):

* At 14 DPO, the average HCG level is 48 mIU/ml, with a typical range of 17-119 mIU/ml.
* At 15 DPO, the average HCG level is 59 mIU/ml, with a typical range of 17-147 mIU/ml.
* At 16 DPO, the average HCG level is 95 mIU/ml, with a typical range of 33-223 mIU/ml.
* At 17 DPO, the average HCG level is 132 mIU/ml, with a typical range of 17-429 mIU/ml.
* At 18 DPO, the average HCG level is 292 mIU/ml, with a typical range of 70-758 mIU/ml.
* At 19 DPO, the average HCG level is 303 mIU/ml, with a typical range of 111-514 mIU/ml.
* At 20 DPO, the average HCG level is 522 mIU/ml, with a typical range of 135-1690 mIU/ml.
* At 21 DPO, the average HCG level is 1061 mIU/ml, with a typical range of 324-4130 mIU/ml.
* At 22 DPO, the average HCG level is 1287 mIU/ml, with a typical range of 185-3279 mIU/ml.
* At 23 DPO, the average HCG level is 2034 mIU/ml, with a typical range of 506-4660 mIU/ml.
* At 24 DPO, the average HCG level is 2637 mIU/ml, with a typical range of 540-10,000 mIU/ml.

Accordingly, the key to interpreting the true value of HCG measurements is to look at their progression. In general, the HCG level will double every two to three days in early pregnancy.

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Doctors can also perform quantitative serum beta HCG levels. This is commonly done in cases of suspected abortion, where a reading that is lower than the expected range of readings based on the patient's stated last menstrual period would be highly suggestive of a miscarriage.

Other uses of a quantitative beta HCG reading are in evaluation of ectopic pregnancy and in trophoblastic and other germ cell tumors.

 

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