On your first week of pregnancy, you're not actually pregnant. Yup... you read that right. Your doctor will count 40 weeks from the start of your last period to determine your baby's due date. However, the baby will not actually be conceived until around the start of Week 3 of pregnancy.
During your period, the uterus sheds its lining. An egg will soon enter your fallopian tube. Should it make a partnership with a fast swimming sperm, this egg will grow into a baby.
If you intend to get pregnant, the start of your period is a good time to kick bad habits out of your life such as smoking and drinking. Although the egg has not yet been fertilized, these unhealthy habits can harm the overall health of your body, the egg and your eventual offspring.