Pregnancy test

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This usually refers to biochemical tests that, in conjunction with appropriate history and clinical examination, can confirm pregnancy. Most tests rely on detecting human chorionic gonadotropin (hCG), a hormone produced by the placenta.

Ultrasonography offers a more definite methods of assessing a viable pregnancy.

Contents

Types of Test

See also Chorionic gonatropin for more details

Urine

Info bulb.pngThe frog test

Frogs (of a convenient species ) ovulate when exposed to hCG. An early pregnancy test involved injecting the woman's urine into the feet of female frogs, and observing their ovaries after an interval. Sometimes known as the Hogben's test.

  • Detects human chorionic gonadotrophin (hCG) in the urine.
  • Available as dipstick test.
  • Immediate answer.
  • Positive or negative (not quantitative).
  • A weak signal can be equivocal, although most modern kits can detect at least 25 mIU.
  • There is a dip in hCG levels from the second trimester onwards (the placenta takes on the role producing different hormones to sustain pregnancy), but the levels are usually still detectable with modern kit.
  • Very high levels of hCG can overwhelm the antibody present in the kit, giving a false negative. This is described as the 'hook' effect, named after the shape of the graph produced from plotting signal to hCG concentration.[1][2][3]
  • Other false negatives may result from mutations/variants of hCG or the presence of gestational trophoblastic disease (rather than true pregnancy).

Blood

  • Requires laboratory analysis.
  • Quantitative.
  • Useful in equivocal cases of suspected ectopic pregnancy. The result should double in 48 hours for a viable intra-uterine pregnancy.
  • Can also be subject to 'hook' effect.

References