How is this possible to have pregnant when someone have sex 3 days to the expected date of menstrual flow.I saw thin mucus(I told this is sign of ovulation) which ended on 19 dec 2012.

How is it possible? Maybe its because few women really know when they actually ovulate... even though many THINK they do... incorrectly.

Because women don't ovulate on a predictable schedule as a group.

Some women can and have gotten pregnant ON their period.