The first official week of pregnancy actually starts on the day of your period. The Estimated Delivery Date (EDD) of your baby is calculated from the first day of your last period. This first week counts as week one of your 40 week (or 280 day) pregnancy, even though you aren't actually pregnant yet!
If you are planning a pregnancy, there are lots of important things to consider. It is recommended you start taking a folic acid supplement (available from your pharmacy) around three months prior to falling pregnant. This reduces the risk of your baby having a neural tube defect such as Spina Bifida.
This is also a good time to think about lifestyle choices and getting ready for baby, making good health, exercise and pre-natal nutrition choices gives you and baby the best start in life. Alcohol can have an affect on fertility so now's a good time to cut back to get your body primed for pregnancy.