The answer depends on how much marijuana is consumed as well as
numerous other factors. The two key facts are these:
1. Marijuana crosses the placental barrier, so whatever marijuana (or
its active ingredients) is in the mother will also be in the baby. For
all practical considerations in terms of marijuana, the mother and the
baby are one organism. (That isn't true for drugs that don't cross
the placental barrier, but most do.)
2. Unlike alcohol and some other drugs, marijuana can stay in the
system for a long time. The general figure used is that marijuana has
a half-life of 36 hours, with some forms of THC (one of the main
active ingredients of marijuana) having a half-life of 72 hours. (Some
sources say the period is longer.) That means that the amount of
marijuana in the system will be reduced in half after 36 hours, will
be reduced to a fourth of the original level in 72 hours, to an eighth
of the original level after 108 hours, and so on. Eventually, of
course, it drops to an imperceptible amount, again depending on how
much was consumed. That can take three or four weeks, even longer in a
Here is some background on marijuana half-life:
Drug Testing: The Half-Life Of Marijuana
"In the infrequent users, the half-life of marijuana was 1.3 days.
That means it took 1.3 days for the amount of marijuana in their blood
or urine to go down 50 percent.
"For frequent users, the half-life was one to 10 days.
"For infrequent users, the time from the last reported drug use until
the last positive specimen was five to 12 days. Frequent users took 22
to 27 days to clear their systems."
How Marijuana Works
The initial effects created by the THC wear off within an hour or two
after using marijuana, but the chemicals stay in your body for much
longer. The terminal half-life of THC is from about 20 hours to 10
days, depending on the amount and potency of the marijuana used.
"Marijuana stays in the system longer than any other drug of abuse.
The half-life (i.e. the amount of time it takes for the body to
metabolize half the drug ingested) is in excess of 72 hours, and in
chronic users it may take weeks to months before it is no longer
detectable in drug tests."
"The GC/MS cutoff level is 15 ng/ml. The elimination half-life of
marijuana ranges from 14-38 hours. At the initial cutoff of 50 ng/ml,
the daily user will remain positive for perhaps 7 to 30 days after
cessation. At the confirmation level of 15 ng/ml, the frequent user
will be positive for perhaps as long as 15 weeks. Marijuana
metabolites' storage and slow release from lipid tissues is the reason
for this long detection period."
And here is one source of many indicating that marijuana passes the
FactLine on Marijuana
"Research on marijuana use during pregnancy is inconclusive, although
like most substances, marijuana crosses the placental barrier.
Evidence suggests that marijuana use during pregnancy may be
sufficient to produce permanent fetal abnormalities, and if combined
with other drug use, inadequate prenatal care or malnourishment, harm
to the fetus may increase greatly."
Like Tutuzdad-ga said, it's impossible to tell you more without
knowing numerous facts such as the potency of the marijuana, the
amount consumed, the amount of fat in the mother's body, the age of
the fetus, and so on and so on. But the basic answer is that it can
take several weeks, more in a chronic user.
I hope this helps.
Search term used: "half life" marijuana
Search term used: marijuana "placental barrier"