Babies start opening their eyes at approximately 26 weeks gestation. At that point their eyelids and eyelashes are fully formed, and they practice using their eyes by blinking and moving them around.
Although their eyes begin development during the fifth week of pregnancy, it takes weeks after birth for a baby's eyes to fully develop. At four months' gestation, their eyes are nearly fully formed. From 26 to 28 weeks' gestation, babies keep their eyes open a lot. They can detect light shining directly into the womb, such as when the mother is sunbathing. By the 33rd week of pregnancy, their pupils constrict and dilate, which allows them to better detect light.