A pregnant woman may notice her first missed menstrual period at the end of the second week after conception, or about four weeks after the first day of her last normal period. There are different kinds of tests for pregnancy. Some may not be accurate for up to three weeks after conception, or five weeks after the first day of the last normal period.
Implantation begins the first week and the embryo continues to grow.
Structures that will become arms and legs, called limb buds, begin to appear. Blood is beginning to be pumped through fetal circulation. Heartbeat is visible by ultrasound. Fingers and toes begin to form. Reflex activity begins with the development of the brain and nervous system.