For anyone trying to get pregnant this is valuable information. 

your pregnancy: pre-conception

“Now that you’ve decided you want to have a baby, it’s time tostart keeping track of each of your periods, not only to know when you’re likely to be ovulating (and thus more fertile) but because once you’re pregnant, the date of your last period marks what doctors call “day #1” of your pregnancy (even though your egg didn’t get fertilized until sometime after then). Doctors will use this date as the jumping off point for due-date predictions and trimester tracking because it’s easier to know the day you started your last period (referred to as the LMP) than to know the moment his sperm met your egg. Including two pre-conception weeks helps to explain why a nine-month pregnancy really stretches to almost 10…..see more…Pregnancy week by week guide – Pre-conception – StumbleUpon.