This happens in the first two 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 twelve weeks, the baby has fingernails and toenails. It is about three inches long and weighs an ounce. Your healthcare provider can let you listen to the baby's heartbeat with something called a Doppler. The second trimester, from thirteen to twenty-four weeks, is when the baby's bones and muscles develop. You will begin to feel the baby's movements.