During the first trimester of pregnancy, the serum HCG rises exponentially, with an increase of at least 66% every two days. With a nonviable pregnancy, the increase over time is less due to decreased production. Comparison in the rise of serial specimens can help decide if the pregnancy is progressing normally or not.
number of days between measurements =
= (((number of hours left in first day) + (number of hours into last day)) / 24) + (number of days in between first and last day)
where:
• hours left in the first day = 24 - (time of day)
• number of days in between first and last day = (last day) - (first day) - 1
(second HCG level) / (first HCG level) >=
>= EXP (0.2534 * (number of days between measurements))
where:
• 0.2534 = LN (1.66 / (2 days))
• 1.66 reflects a 66% increase every 2 days
Interpretation:
• If ratio is >= equation, then the pregnancy is progressing normally.
• If ratio < equation, then the pregnancy may not be progressing normally and the patient needs a more complete evaluation.
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