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Human Chorionic Gonadotropin (hCG) is
also known as the "pregnancy hormone" as it is produced once a woman
becomes pregnant. It can be detected both in the urine and in
the blood,
is produced by the early placenta and keeps the corpus luteum producing
progesterone when conception occurs. The purpose of hCG is to keep estrogen and progesterone
at their appropriate levels until the placenta has developed enough to take over
this
function.
Home pregnancy tests measure hCG in the urine qualitatively,
which means the results shown are either "positive" or "negative".
About 98%
of home pregnancy tests are usually positive around the time of the first missed period
if conception occurred. A blood test measures the hCG quantitatively, which means
that you get a number for the test result. An hCG of less than 5 is usually negative, over 25 usually positive.
At levels above 1,000-1,500 a vaginal sonogram usually identifies the presence of an
intrauterine pregnancy. The blood hCG test is
much more sensitive than a urine test and it can detect pregnancy as early as 10
days after fertilization.
Besides diagnosing pregnancy, hCG is also
often tested for other reasons such as monitoring after a
miscarriage, monitoring an ectopic pregnancy, and after a condition called
hydatidiform mole (or h.mole). HCG monitoring is more useful before the fetal heartbeat is seen (before 6-7
weeks after LMP), usually to rule out an ectopic pregnancy or an early
miscarriage. In an ectopic pregnancy, hCG levels increase at lower rates than in a normal
pregnancy.
HCG is first found in the blood as early as 7-8 days after ovulation
and within the first 2-4 weeks after fertilization, hCG levels usually double every
48-72 hours. There is a wide variation of "normal" hCG levels,
therefore an hCG that doesn't double every 2-3 days does not necessarily indicate a problem.
A maximum level is usually reached by the 10th or 11th week, after which the hCG
level normally decreases somewhat. A dramatic decline in hCG levels may indicate that a miscarriage is taking
place.
Normal hCG values may vary up to 20-times between different pregnancies
and a single hCG test result doesn't give enough information about the viability of
any
pregnancy.
Pregnancies that miscarry or ectopic pregnancies are likely to show
lower levels and slower rises, but they often have normal levels initially. On
the other hand, some normal pregnancies will have very low levels of hCG and deliver
perfectly healthy babies.
Once fetal activity has been detected by ultrasound in a normal patient
population, the chances of normal delivery are at about 95%.
After 5-6 weeks of pregnancy,
sonogram results are much better predictors of pregnancy outcome than are hCG
levels. Once the fetal heart rate is seen, most doctors will monitor the fetal
heart rate instead of drawing blood for hCG tests.
After an hCG injection (e.g., Profasi,
Pregnyl) to trigger ovulation or to
lengthen the luteal phase trace amounts of hCG can remain in the body as long as
14 days or longer after the last injection which may give a false positive
on a pregnancy test. Two consecutive quantitative hCG beta blood tests can
determine whether the hCG is from an injection or pregnancy. If the hCG level
increases by the second test, pregnancy is likely to have occurred.
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