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Prescribing in pregnancy
Drugs can have harmful effects on the embryo or fetus
at any time during pregnancy. It is important to bear this
in mind when prescribing for a woman of
childbearing
age
or for men
trying
to
father
a child.
During the
first trimester
drugs can produce congenital
malformations (teratogenesis),
and the period of great-
est
risk is
from the third to the eleventh week of
pregnancy.
During the
second
and
third trimesters
drugs can affect
the growth or functional
development
of
the fetus,
or
they can have toxic effects on fetal tissues.
Drugs given shortly before term or during labour can
have adverse effects on labour or on the neonate after
delivery.
Not all the damaging effects of intrauterine exposure to
drugs are obvious at birth, some may only manifest later
in life.
Such late-onset effects include malignancy,
e.g.
adenocarcinoma of the vagina after puberty in females
exposed to diethylstilbestrol
in the womb,
and adverse
effects on intellectual,
social,
and functional
develop-
ment.
The BNF identifies drugs which:
.
may have harmful
effects in pregnancy and indi-
cates the trimester of risk
.
are not known to be harmful in pregnancy
The information is based on human data,
but informa-
tion from
animal
studies has been included for some
drugs when its omission might be misleading.
Where care is needed when prescribing in pregnancy,
this is indicated under the relevant drug in the BNF.
Drugs should be prescribed in pregnancy only if the
expected benefit
to the mother
is thought
to be
greater than the risk to the fetus, and all drugs should
be avoided if
possible during the first
trimester.
Drugs
which
have
been
extensively
used
in
pregnancy and appear to be usually safe should be
prescribed in preference to new or
untried drugs;
and the smallest effective dose should be used.
Few drugs
have been shown conclusively to be
teratogenic in humans,
but no drug is safe beyond
all
doubt in early pregnancy.
Screening procedures
are available when there is a known risk of
certain
defects.
Absence of information does not imply safety.
It should be noted that the BNF provides indepen-
dent
advice and may not
always agree with the
product literature.
Information on drugs and pregnancy is also avail-
able from the UK Teratology Information Service.
Tel: 0844 892 0909 (08:30–17:00 Monday to Friday)
Fax:
(0191) 260 6193
Outside
of
these
hours,
urgent
enquiries
only
www.uktis.org
Prescribing in breast-feeding
Breast-feeding is
beneficial;
the
immunological
and
nutritional
value of
breast milk to the infant is greater
than that of formula feeds.
Although there is
concern that
drugs
taken by the
mother might affect the infant,
there is very little infor-
mation on this.
In the absence of evidence of an effect,
the potential for harm to the infant can be inferred from:
.
the amount of drug or active metabolite of the drug
delivered to the infant (dependent on the pharma-
cokinetic characteristics of the drug in the mother);
.
the efficiency of absorption,
distribution,
and elim-
ination of the drug by the infant (infant pharmaco-
kinetics);
.
the nature of
the effect
of
the drug on the infant
(pharmacodynamic properties of
the drug in the
infant).
The amount of drug transferred in breast milk is rarely
sufficient to produce a discernible effect on the infant.
This
applies
particularly
to
drugs
that
are
poorly
absorbed and need to be given parenterally.
However,
there is a theoretical possibility that a small
amount of
drug present in breast milk can induce a hypersensitivity
reaction.
A clinical
effect can occur in the infant if a pharmaco-
logically significant
quantity of
the drug is present
in
milk. For some drugs (e.g. fluvastatin), the ratio between
the concentration in milk and that in maternal
plasma
may be high enough to expose the infant
to adverse
effects. Some infants, such as those born prematurely or
who have jaundice,
are at
a slightly higher
risk of
toxicity.
Some drugs
inhibit
the infant’s
sucking reflex (e.g.
phenobarbital)
while others
can affect
lactation (e.g.
bromocriptine).
The BNF identifies drugs:
.
that
should be used with caution or
are contra-
indicated in breast-feeding;
.
that
can be given to the mother
during breast-
feeding because they are present in milk in amounts
which are too small to be harmful to the infant;
.
that might be present in milk in significant amount
but are not known to be harmful.
Where care is needed when prescribing in breast-feed-
ing, this is indicated under the relevant drug in the BNF.
For many drugs insufficient evidence is available to
provide guidance and it
is advisable to administer
only essential drugs to a mother during breast-feed-
ing.
Because of
the inadequacy of
information on
drugs in breast-feeding, absence of information does
not imply safety.
BNF 61
Prescribing in pregnancy
19
Prescribing in breast-feeding