bbwow,
if your menses fall on 13 Jan 07, by right you can conceive between 24 to 27 Jan (just an estimate), depending on when you ovulate & how long is your cycle. After ovulation, egg can be fertilised between 24-48 hours but sperm can last up to 3 days. so even if you have sex before you ovulate, you still have a chance to get pregnant. and according to tiffany's calculation, if you conceive before ovulation, likely to have girl & conversely will be a boy