Sep 18 2009

Pregnancy Tests and How They Work

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Reading the Home Pregnancy Test instructions before you use it is important There are a number of pregnancies available. A home pregnancy test kit can be self administered in the privacy of your home, a blood test can be administered by a medical facility and tested by a laboratory and an ultrasound scan can be done.

Home pregnancy tests react to the pregnancy hormone, human chorionic gonadotropin (hCG). hCG is produced by the developing embryo (fertilized egg) soon after conception and later in larger quantities by the placenta. Its role is to prevent the disintegration of the ovarian corpus luteum. The corpus luteum forms after the ovarian follicle release the egg. If it does not receive hCG it will decay over the next 14 days and disappear. If it receives hCG from the fertilized egg, it will start to produce testosterone. Testosterone is very important for the woman to get pregnant and maintain pregnancy. This is because the testosterone secreted by the corpus luteum will cause the uterus lining to enrich with a thick lining of blood vessels and capillaries so that it can sustain the growing fetus. Eventually the placenta will take over the production of progesterone and the corpus luteum degrades into a corpus albicans without embryo/fetus loss. hCG is thought to repel the immune cells of the mother, protecting the fetus during the first trimester. It has also been hypothesized that hCG may be a placental link for the development of local maternal immunotolerance. If the immunotolerance does not develop, the uterus will reject the fetus, just like the body will often reject a transplanted organ.

The level of hCG will only be high enough for a home pregnancy test to detect it some time after implantation. For some women this can be as soon as 10 days after ovulation and for some women it can be as late as a day after the missed period. Even pregnancies that implanted may produce too little hCG for a few days after implantation to show up in a pregnancy test. This hCG is also responsible for the pregnancy sign that most women get and get first (aside from a late period) – sensitive and sore breasts.

Between a quarter (25%) and a third (33%) of implanted eggs will only implant for a short while and then fail and abort. The hCG level will stay elevated up to two days after the miscarriage. This is called a ‘chemical pregnancy’ and will often cause a positive pregnancy test followed by a negative pregnancy test a few days later. For this reason it is important to take a second test a few days after a positive pregnancy test.

Home Pregnancy Tests

It is important to follow the instructions exactly for the Home Pregnancy Test, especially concerning the time it should be read A test about 7 days after your missed period is almost guaranteed to be accurate. Any negative test before this time should be viewed with suspicion and repeated 7 days after your missed period to confirm it.

The most reliable and sensitive is First Response Early Result Pregnancy Test. This is according to various tests over many years. It detected hCG at concentrations as low as 6.5 mIU/ml and 12.5 mIU/ml consistently. That’s almost sensitive enough to detect any pregnancy soon after implantation.

Most other kits were far less sensitive – the five least sensitive tests couldn’t detect hCG below concentrations of about 100 mIU/ml at their specified reading times.

‘First Response’, ‘Answer Quick & Simple’ and ‘ClearBlue Easy’ produced result lines that were more intense than others at lower concentrations of hCG, making them easiest to read.

First Response’s Early Result Pregnancy test came out on top as the best combination of sensitivity and reliability. Confirm’s Pregnancy Test scored lowest.

Regular Home Pregnancy Tests:

  • First Response Early Result 12.5 mUI/ml
  • EPT +/- Home Pregnancy Test 25 mUI/ml
  • Clearblue Easy +/1 25 mUI/ml
  • FactPlus Pregnancy Test Stick 25 mUI/ml
  • FactPlus Pregnancy Test Cassette 40 mUI/ml

Store Brands:

  • Target Pregnancy Test Early Result 25 mIU/ml
  • Rite Aid One Step Pregnancy Test 25 mIU/ml
  • Walgreens Early Result Pregnancy Test 25 mIU/ml
  • CVS Early Result Pregnancy Test 25 mIU/ml
  • Wal-Mart’s Equate Pregnancy Test White Cap 25 mIU/ml
  • Wal-Mart’s Equate Pregnancy Test Purple Cap 100 mIU/ml

Seeing the results of the Home Pregnancy Test (HPT) is a special moment Congratulations! Start thinking about baby names

If you are taking hCG hormone treatment (normally as a fertility prescription drug) you need to use a less sensitive test like the Purple Cap Wal-Mart Equate Pregnancy Test

A digital home pregnancy test works the same as a regular test. The only difference is that you do not have to interpret the lines. It will spell out the result as PREGNANT, NOT PREGANANT, YES or NO. The result will stay on the screen for up to 24 hours. Digital test kits are more expensive than regular ones and care should be taken that the battery is still good.

Digital Pregnancy Tests:

  • First Response Gold – 18 mIU/ml
  • Clearblue Easy Digital Pregnancy Test (Blue Cap) – 25 mIU/ml
  • e.p.t Certainty 1-Step Digital Home Pregnancy Test – 25 mIU/ml

Digital Store Brands

  • CVS Digital Pregnancy Test – 25 mIU/ml
  • Walgreens Digital Pregnancy Test – 25 mIU/ml

Home Pregnancy Tests are at least 97% accurate. It normally fails because the user does not follow the instructions accurately. Read the insert that came with the test kit.

  • Use a good quality test
  • Make sure the test is still good (look at the date)
  • Test 7 days after your missed menstruation
  • Repeat the test 2 days later
  • Use the first urine of the day
  • Now how long to wait before reading the test.



hCG is also a marker for tumor marker, in other words, it is a sign of a tumor. It is thus possible to get false positive results from a home pregnancy test because of a tumor. It is not clear if the hCG causes the tumor or is caused by the tumor.

The primary producer of hCG is however the placenta. Before implantation the hCG is present in extremely low concentrations. Super sensitive pregnancy tests have been developed to measure this pre-implantation

Interesting facts about hCG levels:

  • In the beginning of the pregnancy, the hCG level will double every 72 hours in 85% of pregnant women. Further along in the pregnancy as the hCG level gets higher, the time it takes to double can increase to about every 96 hours.
  • The absolute hCG levels are not so important. A normal pregnancy may have low hCG levels and result in a perfectly healthy baby. The results from an ultrasound after 5 – 6 weeks gestation are much more accurate than using hCG numbers.
  • The hCG hormone is measured in milli-international units per milliliter (mIU/ml).
  • Pregnancy Test: hCG level of less than 5mIU/ml – negative. hCG level above 25mIU/ml – positive.
  • The hCG levels should not be used to date a pregnancy since these numbers can vary so widely.
  • A transvaginal ultrasound should be able to show at least a gestational sac once the hCG levels have reached between 1,000 – 2,000mIU/ml. Because levels can differentiate so much and conception dating can be wrong, a diagnosis should not be made by ultrasound findings until the hCG level has reached at least 2,000 (Week 7).
  • A single hCG reading is not enough information for most diagnoses. When there is a question regarding the health of the pregnancy, multiple testings of hCG done a couple of days apart give a more accurate assessment of the situation.
  • There are two types of hCG tests: A qualitative hCG test detects if hCG is present in the blood. A quantitative hCG test (or beta hCG) measures the amount of hCG actually present in the blood.

Guideline to hCG levels during pregnancy in weeks from last menstrual period (LMP) (gestational age):

  • 3 weeks past LMP: 5 – 50 mIU/ml
  • 4 weeks past LMP: 5 – 426 mIU/ml
  • 5 weeks past LMP: 18 – 7,340 mIU/ml
  • 6 weeks past LMP: 1,080 – 56,500 mIU/ml
  • 7 – 8 weeks past LMP: 7, 650 – 229,000 mIU/ml
  • 9 – 12 weeks past LMP: 25,700 – 288,000 mIU/ml
  • 13 – 16 weeks past LMP: 13,300 – 254,000 mIU/ml
  • 17 – 24 weeks past LMP: 4,060 – 165,400 mIU/ml
  • 25 – 40 weeks past LMP: 3,640 – 117,000 mIU/ml
  • Non-pregnant females: <5.0 mIU/ml
  • Postmenopausal females: <9.5 mIU/ml

These numbers are just a GUIDELINE– every woman’s level of hCG can rise differently. It is not necessarily the level that matters but rather the change in the level.

A low hCG level could indicate:

  • Miscalculation of pregnancy dating
  • Possible miscarriage or blighted ovum
  • Ectopic pregnancy

A low hCG level should be rechecked within 48-72 hours to see how the level is changing.

A high hCG level can indicate:

  • Miscalculation of pregnancy dating
  • Molar pregnancy
  • Multiple pregnancy

A low hCG level should be rechecked within 48-72 hours to see how the level is changing.

Doctors does not routinely check hCG levels unless you are showing signs of a possible problem. A health care provider may recheck your levels if you are bleeding, experiencing severe cramping, or have a history of miscarriage.

After a pregnancy loss, most women can expect their levels to return to a non-pregnant range after about 4 – 6 weeks. Health care providers will continue to test hCG levels after a pregnancy loss to ensure they return back to <5.0

Nothing should interfere with your hCG level except medications that contain hCG. These medications are often used in fertility treatments, and your health care provider should advise you on how they may affect a test. All other medications such as antibiotics, pain relievers, contraception or other hormone medications should not have any effect on a test that measures hCG.


The Menstrual Cycle and the hormones controlling it



1. The Best Pregnancy Tests – Consumer Reports Looks At Tests And Ovulation Kits CBS The Early Show Jan 2003 – Consumer Reports

2. Pregnancy Tests Review – ConsumerSearch  Updated Apr 2008 – ConsumerSearch

3, Home test kits: 5 questions to ask before you buy – ConsumerReports   Jun 2008 – Consumer Reports

4. First Response – First Response

5. ClearBlue – ClearBlue

6. e.p.t. – E.P.T.

7. Early Pregnancy Tests . com – Early Pregnancy Tests

8. Home pregnancy tests: Can you trust the results? – Mayo Clinic – Mayo Clinic

9. Your Guide to Pregnancy Tests – WebMD – WebMD

10. Home Pregnancy Tests HCG Levels and FAQ – Fertility Plus, An evaluation of HPT sensitivity

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