A full-term pregnancy may have two opposing effects on breast cancer
risk: an adverse one shortly after delivery and a beneficial one in
the long run. We propose to study the postpartum time-varying breast
cancer risk by comparing two adjacent parities at a time and
examining current age as a modifier of the effect of age at delivery.
Effect modification can be assessed by stratified analysis or
modelling with interaction terms. For a case-control study with a
moderate sample size, the modelling approach treats age and age at
delivery as continuous variables under the assumption of linearity in
logit of risk. Applying the model with interaction terms, a
transient increase in breast concern risk lasting for about 15 years
after a childbirth has recently been reported in a large population-
based study. A linear model would fail to reveal any short-term
excess in postpartum breast cancer risk that might peak several years
after the delivery. A large database which links together the
Swedish Fertility Registry and national Cancer Registry will be
utilized to determine, using single-year indicator variables for age
at delivery and stratifying by current age in one-year intervals,
whether there is a postpartum period when the risk of breast cancer
is the highest. We then evaluate whether the model with the
linearity assumption yields effect estimates similar to those derived
from the procedure using categorical variables. Other questions to
be studied include whether the transient increase in risk varies
significantly with age at the pregnancy, and whether any short- or
long-term effect of a multiple birth on breast cancer risk differ
from that of a singleton birth. Included in the analysis will be an
estimated number of 34,000 breast concern cases that occurred in
Sweden between 1958 and 1994 and 170,000 age-matched population
controls, randomly selected from among those in the source
population, alive at least to the date of diagnosis of the index
case, and who had not been diagnosed with breast concern by that
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