Pregnancy Tests


Written by : Dr.M.D.Mazumdar, MD

The aim of all pregnancy tests in the blood and in the urine is the identification of the Human Chorionic Gonadotrophin (HCG) hormone by the use of special reagents.

With the invention of pregnancy home test kits, this identification has become easier and less expensive.

Human Chorionic Gonadotrophin (HCG)

HCG was first detected in human urine by Ashcheim and Zondek in 1927. In the 1930's, it began to be used to diagnose pregnancy by injecting pregnant human urine into animals like mice and rabbits.

With better knowledge about the procedure, nowadays, inert latex particles coated with antibodies to HCG are widely used for pregnancy tests.

HCG is secreted by the human placenta as early as 7 to 10 days after fertilization, that is, at around the time of implantation. At this time, it can be detected by special radioimmunoassay tests in the blood, but not in the urine. The level rises gradually and HCG becomes easily detectable in the urine at around 2 weeks after fertilization, that is, at the time of the first missed period. Recent improvements have made it possible to diagnose pregnancy even earlier, at about 10 days after ovulation, that is at 3-4 days before the missed period.

Depending on whether HCG is present in the urine or not, the results of a pregnancy test may be positive or negative. The daily level of HCG is difficult to estimate unless the total urine output in 24 hours is collected and measured. The level of HCG is highest in the first urine sample passed in the morning since the urine collects in the bladder overnight. Later in the day, the level of HCG may vary from time to time depending on the amount of fluid drunk by the woman.

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Examination for HCG in the serum (blood) is diagnostic not only of the presence of HCG but also the level of HCG in the serum. (Serum is formed when blood is allowed to remain motionless in a vial. The solid parts of blood like the RBC's, WBC's and the platelets get deposited at the bottom of the vial. The clear straw coloured fluid that remains at the top is the serum).

HCG starts to be released into the blood stream soon after implantation of the embryo into the endometrium. The level at this time is very low - about 5mIU per ml. But it increases very fast, doubling every 2-3 days to reach 50 - 80 IU per ml at about the 4th week of pregnancy. This level is high enough to cause some HCG to be excreted in the urine and become detectable.

The level of HCG peaks at around the 12th week of pregnancy and then stabilizes. This is maintained till the end of the pregnancy. HCG completely disappears from the urine within 2 weeks of delivery.

The level of HCG may vary from women to women and from pregnancy to pregnancy in the same women. What is more important than the level of HCG is whether the level is rising or decreasing. If a blood test is repeated after 3 days of the first test and the level shows a rise, then the pregnancy is definitely progressing. If however the level shows a lower level than the first, then there could be a risk of an abortion.

MEAN VALUES OF HCG SERUM

Weeks of Pregnancy Range of HCG level Average HCG level(mIU/ml)
Week 3(at the time of implantation) 0 - 5 I.U. 48 I.U.
Week4 (at the time of the first missed period 50 - 500 95
Week 6 1000 - 50,000 3000
Week 8 7000 - 200,000 10610
Week 12 25,000 - 300,000 29,070
Week 20 4000 - 100,000
Week 40 (at the time of delivery) 1000 - 50,000
Week 42 (2 weeks after delivery) 0 - 5

Functions of HCG:

  • It stimulates the secretion of progesterone from the corpus luteum of pregnancy present in the ovary.

  • It suppresses the immune system of the mother, preventing it from rejecting the foetus as a foreign body.

  • It stimulates the production of fetal hormones.

TESTS FOR HCG IN THE URINE

Earlier, tests for HCG in the urine was done on the 8th day after the first missed menstrual periods. But now, with tests becoming more reliable, HCG can be detected in the urine as early as the first day of missed menses.

Some pregnancy test kits can detect HCG even before the day of the missed periods.

Advantages of Home Pregnancy Kits

  • Tests are simple and can be easily done by anyone at home.
  • It can be done privately.
  • Results are obtained very fast.
  • Most tests are 99% accurate.
  • Most tests are quite cheap.

Methods of Testing for HCG

There are different types of home pregnancy tests.

  • Slide test: A drop of urine has to be placed on a slide containing an antibody to HCG. A visible change is produced if HCG is present in the urine sample and the test is then declared positive.

  • Dipstick test: A stick containing the antibody is either dipped into a container of urine or held in the urine stream. A positive result produces a visible change on the stick.

  • Digital Pregnancy Tests: These tests consiti of a slide on which a drop of urine needs to be placed and the text 'Pregnant or 'Not Pregnant' shows up after some time. Unlike the other tests which show lines, there is less scope for misreading the report.

TESTS FOR HCG IN THE BLOOD

  • Radioimmunoassay: This highly sensitive test can detect minute quantities of HCG in the blood - as low as 0.003 I.U. per ml of serum.This test can be done 7 - 10 days after fertilization, that is, at the time of implantation, for a positive result.

  • Radioreceptor Assay: can detect levels of 0.001 I.U. per ml of HCG in the serum.

  • ELISA Test: Extremely small levels of HCG can be detected by this test - as low as 50 m I.U. per ml of serum.

False Negative tests:

Sometimes the tests may declare the absence of pregnancy although the woman is pregnant. This can happen in:

  • Ectopic Pregnancy: When the level of HCG produced is low as compared to the duration of pregnancy.

  • When the antibody in the kit has degenerated due to poor maintenance. Most kits need to be kept at 2 degrees to 10 degrees of temperature.

  • When the test has not been carried out properly. The reading of the slide or the dipstick has to be done after 2 minutes of the test and not later than 10 minutes.

False Positive Tests:

Sometimes the tests may be positive, although the woman is not pregnant.

  • Certain tumours of the ovary can produce HCG, giving a false
    positive test.

  • Certain tumours of the testes in men can produce HCG, giving a false
    positive test.

IN THE FOETUS:

HCG can also be detected in the foetal blood in the early weeks of pregnancy. But like the levels in the maternal blood, the level of HCG in the foetal blood also gradually decreases by the 20th week.

Also Read-

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