Your little embryo begins to grow by leaps and bounds, even though that might be hard to believe when it’s only about the size of a grain of rice. (Embryos and fetuses are most often measured from crown to rump because the baby’s legs are generally bent and hard to measure.) The embryo doesn’t quite look human yet: it resembles a tadpole that’s curled up like a comma and has what appears to be a tiny tail. That “tail” is actually the end of the developing spinal column.

The embryonic brain and heart are in the forefront of growth this week and will soon occupy about half of the entire embryonic body. The brain is developing three separate parts: the forebrain, midbrain, and hindbrain. The head is growing larger to accommodate all that growth. Around 28 days after fertilization, the early divisions of what will be the heart’s four chambers are apparent. Just a few days later, the four chambers are fully formed. The tiny buds of the lower and upper limbs appear, too.