The epidural space is not a completely sequestered compartment in the
mother's body.  If it were, the epidural effects would be permanent.  The
drugs enter the maternal circulation and are metabolized, normally by the
liver, and while in circulation can enter the fetal circulation as well.
Some drugs are more rapidly metabolized than others, but all drugs are more
rapidly metabolized by mother's liver than by the baby's.  Any drugs still
in the fetal circulation when the fetus turns into an infant with its own
respiration and waste excretion, separate from mother's, will last as long
as it takes for the baby to metabolize them.
Also, some of the metabolites of analgesics are themselves analgesic in
effect, and are longer lasting.  
Rachel Myr
Kristiansand, Norway

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