Carroll: If you go to the original article, it's conclusion is that this study was too small to be conclusive--that at this time no particular conclusion can be drawn.
http://www.bmj.com/cgi/content/full/329/7479/1375
I guess in the end it comes down to whether a person could be very confident they were getting enough folate in diet for those few months that are so important, since it does seem quite certain that folate even in supplement form decreases the risk of neural tube defects. If a person was very confident they were getting enough dietary folate then it would not seem necessary to take a supplement. However, if one weren't certain, weighing the health of a babe now compared to risk of breast cancer later is tough--especially since we have no idea of the diet or lifestyle of the women in this study between when they entered the study and when mortality was examined (I believe they were just screened for weight, smoking, etc upon entry, not intervening years where risks for breast cancer could have drastically changed).