Each trimester lasts between 12 and 14 weeks, or about three months. Each trimester comes with its own specific hormonal and physiological changes. Being aware of the ways that your growing baby is affecting your body will help you to better prepare yourself for these changes as they happen. Pregnancy date counting starts with the first day of your last normal menstruation cycle, and conception takes place in week 2. The first trimester lasts from the first through the 13th week of pregnancy.