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2.b Why is the age of the fetus important?




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This article is from the Pregnancy Screening FAQ, by Lynn Gazis-Sax (gazissax@netcom.com) with numerous contributions by others.

2.b Why is the age of the fetus important?

2.b The normal ranges of AFP and the hormones of the triple screen will
vary with the age of the fetus. So in order to determine if the values are
in the normal range, one must have an accurate date. This can be
problematic if date of conception or last menstrual period is not known.
For women with cycles significantly longer or shorter than 28 days, or who
don't have regular cycles at all, scheduling or interpreting the results of
the screens will be difficult if not impossible without ultrasound to more
accurately date the fetus.

The first thing that many doctors do when a patient has an abnormal AFP or
triple screen is request an ultrasound to recalculate the age of the fetus
(and to rule out twins). Often the recalculated age is enough evidence to
rule the screen result normal.
-----------------------------

Addition by Dr. Tim Reynolds:

Better to do scan first (this is standard practice in most centres in the
UK).
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