Conception occurs about 2 weeks before your period is due. That means you may not even know you're pregnant until you're more than 3 weeks pregnant. Yet your baby is most sensitive to harm 2 to 8 weeks after conception. This is when your baby's organs (such as the heart) begin to form. Anything you eat, drink, smoke or are exposed to can affect your baby. That's why it's best to start acting as if you're pregnant before you actually are.