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The protective effect  of an early first full term pregnancy against future development of breast cancer is has been undisputed  for 35 years.

The landmark study establishing this protective effect [MacMahon, et al, (1970) Bulll WHO 43:209-221] is widely accepted in the medical world.  

MacMahon, and group reanalyzed their 1970 data [Trichopolous D,  Hsieh C, MacMahon B, Lin T, et al,, Age at any Birth and Breast Cancer Risk, International J Cancer, 1983:31:70l-704], finding that each one year delay in the first full term pregnancy increased relative breast cancer risk by 3.5% (compounded). 

Obviously, aborting a first pregnancy eliminates the protective effect against breast cancer.

In an attempt to calculate the risk in numbers that both patient and physician can more readily relate to, Thorp applied the Gail model risk analysis to typical real-life situations (Thorp et al., Long Term Physical and Psychological Health Consequences of Induced Abortion: Review of the Evidence; Obstetrical and Gynecological Survey, Vol 58, #1, Jan 2003, pp 75,76).  The following discussion, including table 8, 9, and 10, are excerpted directly from this reference. 

“We think, given the undisputed protective effect of a full-term delivery early in one’s reproductive life on subsequent breast cancer development, that a young woman facing an unwanted or crisis pregnancy can and should be informed of the loss of that the loss of
that protection that would derive from her decision to terminate her pregnancy and delay having a baby  (98, 101). 

“To illustrate, Table 8 uses the Gail Equation to predict 5-year and lifetime risk of breast carcinoma for an 18 year-old woman with an unintended or crisis pregnancy.  The Gail model (99) is considered the best available measure for estimating an individual woman’s risk of developing breast cancer.
“In the first scenario, she decides to terminate and then has her first term delivery at age 32;  in the second scenario, she has a live-born infant.  We then assess her individual risk at age 50, when the risk of breast cancer begins to peak.  For both black and white women, her decision at age 18, and her subsequent reproductive choices, can almost double her 5-year and lifetime risk of breast neoplasia at age 50.  Tables 8, 9, and 10 demonstrate that the “loss of protection” effect is most pronounced in women under 20 years of age who elect to undergo abortion rather than continue their pregnancy. 

“We think, now, that clinicians are obliged to inform pregnant women that a decision to abort her first pregnancy may almost double her lifetime risk of breast cancer through loss of the protective effect of a completed first full-term pregnancy earlier in life.”

(References are not reproduced here, but are available in the Thorp et al. original article)
[Joe DeCook, MD for AAPLOG]