| |
2004/12/29 04
Overweight women were 60% more
likely to fall pregnant while on the Pill.
Obese women were
70% more likely, found a study
in Obstetrics and Gynaecology by a team from the Fred Hutchinson
Cancer Research Centre in Seattle.
It suggested that of 100
women on the Pill, an extra two to four would fall pregnant
due to being overweight.
The Pill is usually estimated to be
over 99% effective. This means that less than one woman in
100 will get pregnant in a year.
However, that figure relates
to perfect Pill use. Actual failure rates are estimated to
run at around 6%.
Researchers compared the weight and body-mass
index (BMI) of 248 women who became pregnant while on the Pill,
and compared them to a group of 533 women of the same age who
were taking oral contraceptives but who were not pregnant.
BMI is calculated by dividing your weight
in kilograms by the square of your height in metres. A BMI
of over 25 is considered overweight and one of 30 or above
is considered obese.
It was found that the link between carrying
extra weight and contraceptive pill failure became evident
in women whose BMI was 27.3 or higher - equivalent to a 5ft,
4in woman who weighs 160lbs (11st 6lb, or 72.5kg) or more.
Complications
The researchers say their study did not look
at why this link should exist. But they suggest a higher metabolism,
linked to extra weight, could be a factor, because it can shorten
the duration of a medication's effectiveness, or that hormone
levels in the Pill may not be high enough for larger women.
In addition, they said the more overweight or obese a woman
is, the more liver enzymes they have to clear medications from
the body, causing a drop in the amount of a drug circulating
in the blood.
The researchers say another explanation could
be linked to the fact that the active ingredients in oral contraceptives,
oestrogen and progesterone, are stored in body fat - so the
more likely the drug is to become trapped in the fat instead
of circulating in the bloodstream.
'Health hazard'
Dr Victoria
Holt, who led the study, said: "These results represent yet
another reason why obesity is a health hazard.
"Overweight
and obese women have a significantly higher risk of getting
pregnant while on the Pill than women of normal weight, and
this translates into a substantial number of unplanned pregnancies."
She
added: "This higher risk of pregnancy also translates into
a higher number of obesity-related complications of pregnancy,
which range from gestational diabetes and high blood pressure
to Caesarean delivery."
Dr Holt said women who are overweight
should not ask for a higher dose Pill, because they were already
at a higher risk of cardiovascular disease which contraceptive
hormones increase even more.
She suggested women who had completed
their families should consider a permanent form of birth control
such as sterilisation, and those who still wanted to have a
family considered using a back-up form of birth control, such
as a condom, as well as the Pill.
She added: "I think losing weight, if one is substantially overweight, is a terrific
idea for many health reasons and a laudable goal."
But she said, "I also acknowledge
that it is often difficult to do."
Geoffrey Chamberlain, professor of obstetrics
and gynaecology at the University of Wales College of Medicine, said: "The Pill
is not so effective in overweight women. The hormones get absorbed into the fat,
so the blood concentration and the effect on the ovaries is lower.
"Therefore,
it may be advisable for women who are overweight to use other methods of contraception
such as barrier methods or an intrauterine device."
BBC
NEWS
© BBC MMIV
|
Contraception