Your first clue: Your menstrual period is late! (Dating a pregnancy actually starts from the first day of your last period, so by the time you miss your period you're considered four weeks pregnant.) You also may have swollen, tender breasts; up to 5 pounds of extra weight (much of it water); deep fatigue; mild to extreme nausea any time of the day or night (with or without vomiting); food cravings and aversions.
Once your pregnancy is confirmed, it's natural to swing from elation to apprehension about potential complications to conjecture about motherhood and changes in your relationship, your job and the rest of your life.
At week four, your baby has progressed from being a cluster of cells called a blastocyst to an embryo that's about the size of a pencil point. The neural tube (from which the brain, spinal cord, spinal nerves and backbone develop), heart and other organs are beginning to form. This is when it's especially important to be careful about what you put in your body.