@ 27 Weeks

January 21-28, 2007. Something both frightening and amazing occurs this week: the third trimester begins. The baby's still quite small, just over two pounds, but he'll only grow a few more inches in total length before he's born--he's already more than 15 inches long; he'll mostly put on weight and fill out more between now and April, his head will grow a bit more as the brain continues to develop, and his organs will continue to grow. Most fascinating this week in the baby's development is that the eyes' multiple layers of light-sensitive cells--the retinas--and the nerves connecting the cells to the brain are now developed. More amazing stuff: the eyes, or what become the eyes, first appeared as little grooves on the side of the blastocyst (the very early embryo) just a month into the pregnancy. Developing from the ectoderm--the part of the early cellular "glob" that becomes the brain, nervous system, skin, and the hair--they grew and changed into pockets of rich, specialized cellular material called optical vesicles. At about seven weeks, the eyes began moving from the side of the head to the middle of the face, and binocular vision became possible. Amazing, huh? At about nine weeks, the pupils and optical nerves connecting the eyes to the brain formed (you won't be able to see this, but if you look at the photo in the @ 9 Weeks entry below, you'll have a sense of how early this occurs). With weeks 11 and 12 came eyelids. The eyelids remained fused shut, as they do in many other animals, until the light-senstive cells essential to vision were well-developed. In humans this occurs in the 27th week. So this week Peanut's eyelids will no longer be fused shut. He may open and close his eyes, he'll be able to respond to light, and his little brain can process light information: he can see.