This is definitely unlike any of the other postings that I've done on here. It is my take on one article covering the study of the link between breast cancer and having a child with Down syndrome. Some of my comments may make no sense as they are responding to questions that other moms have asked on facebook.
This is the article that I read. I don't actually have a physical copy as I read a friend's copy.
Bjørge T, Cnattingius S, Engeland A, Tretli S, Lie RT, Lukanova A.
Epidemiology. 2009 Jul;20(4):584-9.
This study is actually pretty amazing. It is a much more solid study than I expected. First of all, the study does have a few weaknesses that they outline in the article, but as the study was meant to study the link between Human chorionic gonadotropin (hCG) and breast cancer and not Ds and breast cancer, most of the weaknesses of the study really don't have much to do with the link between Ds and BC. The other weaknesses are the difference in age and the possible difference in breast feeding and surprisingly the average age was only off by two years which I can't imagine would affect the amount of breast cancer by 25%. As for the breast feeding there is really no telling but it is also unlikely have such a huge difference. Studies show a protective effect, but only in regards to those that have one of the breast cancer genes BRCA1 (as opposed to BRCA2 and breast cancers without the genes) and even then the percentage wouldn't be this high in this large a group, unless these countries were the origin of that breast cancer gene mutation - possible for one of the mutations of the gene, but not all of them. If it were the place of origin, the 25% increase would still be there, but only limited to mothers that have the gene and choose to not breastfeed in regards to this study and would still be high in other areas with other mutations. It's unlikely this is the case, but a lot of gene mutations have small focal origin points like this. BRCA1 has various mutations occur from various countries, so it is likely to be higher not lower in the US, where we have multiple mutations of the gene to choose from.
There are some weaknesses that I see that they don't list. One weakness that I identify, but that isn't in the study, are the women that had miscarried or had an abortion of a child with Ds and later got breast cancer, which would make the link stronger not weaker because the moms would have been included in the wrong side of the study. Also are children that weren't identified as having Ds at birth, but that might have died from heart or intestinal (or other issues) before ever getting a diagnosis, which if were known to be included could also have made the link stronger. There is no way that they could collect that data though.
I was also skeptical of their 95% confidence level because I couldn't imagine how they had gotten enough moms to get that statistical surety, but I didn't realize they had almost complete data from 2 whole countries.
As for the age 50 thing, I have some bad news. It looks to me that they were only looking for their statistical analysis at women that were under 50 when they found their first cancerous tumor, not that the only increase was for that age group. If you look at the actual data they posted, The breast cancer numbers actually increased with the time from the birth of the child with Down syndrome and since the age of the mom's in the study (not the ones for the statistics) went well past 50, the cancer numbers still show up in the raw data.
I find it interesting and frustrating that this study wasn't even about mothers of Ds, but actually it was just convenient to study us for their purposes because of the hCG connection. It just shows how none of our questions are likely to be answered unless we happen to be convenient as a study point of something else (like the way our kids are used for cancer prevention studies).
I might have an epiphany when thinking about this as I drift to sleep tonight as I often have things come into focus then that didn't occur to me earlier. If so, I'll post those thoughts in this post.