Your baby's head is as large as the rest of his body, but his arm and leg buds are starting to lengthen. His nose is starting to show, and eyelids are forming. Your baby's eyes already have a lens at this point, as well as an iris, cornea, and pigmented retina. Your baby's spine is completely developed and has closed over at both ends. Inside his lungs, secondary branches have started to appear, and they will continue branching out after he's born. Someday these branches will connect to the 300 million tiny air sacs (called alveoli) in his lungs.
Although your blood is supplying nutrients and oxygen to your growing baby, your blood does not flow directly to him. Instead it passes through the umbilical cord to the baby via the placenta, a disk-shape organ with lots of little roots and veins. The placenta produces hormones that support the baby's growth; it also sends nutrients and oxygen to the fetus and helps excrete the baby's waste. Think of it as a glorified filter that prevents your bloodstream from ever directly mixing with your baby's; because of this filter, your baby can have a blood type that's different from yours.
Originally published in You & Your Baby: Pregnancy.
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