 Researchers
have found a key risk that could quite literally add to
the hefty burden on overweight and obese women's lives
— unplanned pregnancy. A startling study in the
January issue of Obstetrics & Gynecology reveals
that such women are far more likely than others to be
let down by their oral contraceptives (OCs).
Lead author Dr Victoria Holt, an
epidemiologist at the University of Washington in Seattle,
says the root of the problem is simple — the Pill
has been getting smaller, while women have been getting
bigger. Today's Pill often contains just 20% as much
estrogen as the formulations of a generation ago.
BMI
BABY
A number of public health institutes in Washington state
collaborated to conduct a case-control study of 248
women who became pregnant while using OCs between 1998
and 2001, and 533 age-matched women who used the Pill
during the same period but didn't get pregnant.
They found that the risk of pregnancy
was nearly 60% greater in women who had a body mass
index (BMI) over 27.3. In terms of the official classification,
such a BMI is at the mid-point between overweight (BMI
25+) and obese (BMI 30+).
When women with a BMI of 32.2 or
more were considered alone, their risk of Pill failure
was 72% higher than in women with a BMI lower than 27.3.
The findings were even clearer when the analysis was
confined to those who never skipped a pill. In this
category, subjects with a BMI over 27.3 were more than
twice as likely as those with a lower BMI to become
pregnant despite taking OCs.
According to Health Canada, 27.5%
of Canadian women aged 20-64 have a BMI over 27. And
since the Childbirth by Choice Trust says 76% of women
in this country use contraception, with 27% relying
on the Pill, the risk is significant. Half of Canada's
estimated 40,000 annual unwanted pregnancies occur in
women using contraception.
WEIGHT-TO-DOSE
RATIO?
The researchers got the idea to test the Pill in overweight
women at an FDA meeting several years back, when the
agency was considering removing higher-dose pills from
the market. One doctor on the panel spoke out: "Don't
do that. All of my overweight patients are going to
get pregnant."
It's not just that bigger people
need bigger doses, explains Dr Holt. The association
between OC failure and obesity as measured by BMI was
much stronger than the association with unadjusted weight.
In other words, a woman who weighs 73kg and measures
1.6m (160lb/5'4") is more likely to get pregnant than
a 73kg woman who's 1.8m tall (160lb/5'10").
Dr Holt suggests three likely mechanisms.
First, overweight and obese people have a faster metabolism,
which may mean the Pill is used up more quickly in these
women. Secondly, overweight women have more liver enzymes,
which help metabolize the hormones in OCs. Finally,
she says, "women with high BMI are more likely to have
more fat and the hormones in birth control pills are
fat-soluble. The hormones may get trapped in the fat
rather than entering the bloodstream."
EXTRA
PROTECTION
The total extra risk is small but not negligible. Dr
Holt puts it at an additional two to four pregnancies
per 100 woman-years of use among overweight women. But
she's opposed to a return to high-dose formulations,
pointing out that the Pill's strength was reduced to
avoid cardiovascular complications that pose a particular
menace to overweight women. If patients have already
completed their planned families, she suggests they
should consider sterilization, and if not, they could
back up their OC with a barrier method such as condoms.
That may not be such a bad idea
regardless of weight or STD risk, as it may be that
our optimistic assumptions about the Pill's efficacy
don't hold true in this era of low doses. For years,
the commonest figure given for the Pill's one-year failure
rate has been about 1%, but this is based on perfect
compliance with the old, high-dose pills. The US National
Survey of Family Growth, considered a very reliable
guide to real-world behaviour, puts today's failure
rate at 5-7%. In an earlier pilot study, Dr Holt and
colleagues found that about 6% of overweight women on
the pill experienced unwanted pregnancies in a 12-month
period, and even in women of healthy weight, the failure
rate was over 2%.
Obstet Gynecol 2005;105(1):46-52
|
