We all know two things that affect our health as adults: how we live
our lives (diet, exercise, stress reduction, exposure to toxic compounds
and the like) and the genetic endowment we got from our parents (genes
that raise the risk of cancer, for instance). But over the last few
years scientists have begun to recognize a third factor: conditions in
the womb during our nine months of gestation. Now comes more support for this idea. In a new study published today
in PLoS Medicine, scientists report that birth size, and in particular
birth length, correlates with a woman’s risk of breast cancer in
adulthood. Birth size has long been recognized as a marker of the prenatal
environment. In 1999 I wrote the first article in a general publication
about the role that the prenatal environment plays in people’s risk of
“adult-onset” disorders such as heart disease, stroke, cancer, diabetes
and other diseases that we think of as the product of genes or
(postnatal) environment. As I said at the time, “The health we enjoy
throughout our lives is determined to a large extent by the conditions
in which we developed, [conditions that] can program how our liver,
heart, kidneys and especially our brain function. It is no exaggeration
to call these findings a revolution in the making. The discovery of how
conditions in the womb influence the risk of adult disease casts doubt
on how much genes contribute to disease (because what scientists
classify as a genetic influence may instead reflect gestational
conditions) and suggests that adult illnesses long blamed on years of
living dangerously (like dining on pizza and cupcakes) instead reflect
‘fetal programming.’ ‘Two years ago no one was even thinking about
this,’ says Dr. Matthew Gillman of Harvard Medical School. ‘But now what
we are seeing is nothing short of a new paradigm in public health.’" That paradigm has gotten stronger. In the new study, scientists led by Isabel dos Santos Silva
of the London School of Hygiene & Tropical Medicine examined 32
previous studies on the correlation between birth size and breast
cancer. That gave them 22,058 cases of breast cancer from a pool of more
than 600,000 women. Their conclusion: birth weight is positively
associated with breast cancer risk in studies where information on birth
size was based on birth records (not in those based on adult
self-reports, which are less accurate). Specifically, every 0.5 kg (about 1 pound) increment in birth weight
was associated with about a statistically-significant 7% increase in the
risk of breast cancer. Relative to women who weighed 6.6 to 7.7 pounds
at birth, those who weighed 8.8 pounds had a 12% greater risk of
developing breast cancer. Birth length and head circumference were also
associated with an elevated risk: for body length at birth, for
instance, the incidence of breast cancer by age 80 is 10%, 10%, 10.4%
and 11.5% in those who were in the bottom, second, third and top
quarters of birth length, respectively. In other words, the longest baby
girls had a 15% greater risk, but that still put them at only 11.5%
compared to 10%. That is certainly not huge. (It’s comparable
to the increased risk of breast cancer from drinking alcohol.)
Nevertheless, “our study indicates that birth size is a marker of
susceptibility to breast cancer in adulthood,” dos Santos said. “The
birth size-breast cancer association appeared to be largely independent
of known risk factors,” such as diet, pregnancy and nursing. And the
association with birth weight held up even after the scientists made
statistical adjustments for the woman’s body-mass index in adulthood
(obesity is a risk factor for breast cancer), for her mother’s size and
other possibly confounding factors. In all, the scientists estimate,
large size at birth accounts for about 5% of breast cancers in the U.S.
and other developed countries. In an expert commentary on the study, Pagona Lagiou and Dimitrios Trichopoulos
of Harvard University School of Public Health write that it provides
“the strongest evidence yet that birth size is a critical determinant of
breast cancer risk in adult life.” (Trichopoulos, way back in 1990,
suggested that a baby girls’ size at birth might affect her risk of
developing breast cancer as an adult, reasoning that in utero exposure
to estrogen—which promotes growth—might also primate breast tissue to
make it more likely to develop cancer.) |