MULTIPARITY RISK
XING ET AL. 2009
Xing et al. found an increase in risk associated with having more
children (multiparity) among women who delayed FFTP and never
breastfed. The increased risk probably resulted from confounding due to
the temporary increase in risk from full term pregnancies. Childbearing
is protective over the long-term, but studies confirm there is a small,
temporary risk among multiparous women who delay FFTP until after age
25. [1] The study, Henriet et al. 2001, shows that induced abortion
contributes to this problem. They reported that women with no abortions
have double the odds of having a FFTP before age 25 compared to women
with multiple induced abortions. [2]
How can multiparity increase risk among women who delayed their FFTPs?
Breast cancer risk increases with age at FFTP. A delayed FFTP lengthens
the period between puberty and FFTP known as the "susceptibility
window" when nearly all of the breast lobules are cancer-susceptible
and exposed to the cancer-causing effects of estrogen increases during
monthly menstrual cycles, resulting in an accumulation of the effects
of cancer-causing substances.
Then, during a normal pregnancy, breast growth takes place. Pregnancy
hormones (primarily estrogen) stimulate dramatic cell growth that can
induce a precancerous cell to grow into a cancer cell, which can grow
into a malignant tumor. During the last months of FFTP, 85% of the
mother's cancer-susceptible breast lobules mature into fully
cancer-resistant lobules.
The transient increase in risk from multiparity and late FFTP is
associated with premenopausal breast cancer, and it lasts about 15
years after the last birth. If the mother escapes that risk, then she
will be left with fewer places for cancers to start in her breasts and
she will be better off in terms of long-term breast cancer risk. The
overwhelming majority of breast cancers occur at age 50 or older. Over
the long term, having a full term pregnancy is always protective.
In the study, Xing et al., the transient risk shows up in the
population because it is a population of women who are relatively young
as breast cancer patients and relatively old when they had their first
child (age 25 and over). If Xing's team keeps following the controls to
see who gets breast cancer over the next ten years, they will probably
find this short-term risk disappears. The full protective effect of
childbirth is long-term, and in this population, they're looking at a
relatively short term between last childbirth and diagnosis.
References:
1. Liu Q, Wuu J, Lambe M, Hsieh S-F, Ekborn A, Hsieh C-C. Transient
increase in breast cancer risk after giving birth: postpartum period
with the highest risk (Sweden). Cancer Causes and Control 2002;13:
299–305.
2. Henriet L, Kaminski M. Impact of induced abortions on subsequent
pregnancy outcome: The 1995 French national perinatal survey. BJOG
2001;Oct;108(10):1036-42.
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