This happens in the first 2 weeks. Even before you know you are pregnant, the baby's nervous system, heart, and lungs have begun to develop. The first trimester starts with your last menstrual period. Conception happens about two weeks after your period. Eight weeks after your last period, your baby's brain and spinal cord are developing, and its heart is beating.
First trimester is an important time in the baby's development.
By the end of the first trimester at 12 weeks, the baby has fingernails and toenails. It s about three inches long and weighs an ounce. Your healthcare provider can let you listen to the baby's heartbeat with a special instrument. The second trimester, from 13 to 24 weeks, is when the baby's bones and muscles develop. You will begin to feel the baby's movements.