The first trimester in pregnancy is from week 1 until week 12. In the first few weeks your body is undergoing many changes and adapting to the different hormones. During this time your baby is forming with all the major organs and body structures. By then end of 12 weeks your baby is the about 3 inches long.
Watch the March of Dimes Videos on the month-by-month growth of your baby:
Baby growth month 1
Baby growth month 2
Baby growth month 3
What happens during the 2nd Trimester?