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hCG is detectable in the blood serum of approximately 5% of pregnant women by 8 days after conception, and in virtually all the rest at 11 days. hCG rises progressively from conception. Levels double on the average, every 30.9 hours until values reach 6500 mIU/ml (6,500 IU/L) at approximately the eighth week after the last menstrual period (LMP). After that the rate of rise becomes individualized, peaking between the 60th and 70th day (9 to 10 weeks) LMP.
hCG decreases slightly between the 12th and 16th-week, and then remains constant until birth. A blood serum level of less than 5 mIU/ml (5 IU/L) can be considered negative and anything above 25 mIU/ml (25 IU/L) positive for pregnancy. If you are unsure, repeat the test in two days to see if there is an upward trend, which indicates pregnancy. Levels higher than expected for the weeks pregnant may be due to multiple gestation or inaccurate dates. Levels in normal pregnancy fall within the following wide ranges.
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