Pat yourself on the back: You're in the midst of a truly mind-boggling feat! Although it's only the size of a sesame seed at this early stage, your baby is growing at an astronomical rate.
Currently, he's sporting three distinct layers called the ectoderm, mesoderm and endoderm. The top layer, or ectoderm, will soon house the baby's neural tube, which produces the brain, nerves, spinal cord and other related features. The ectoderm will also eventually develop the baby's skin, tooth enamel, hair and glands.
The middle layer, or mesoderm, is where your baby's tiny heart is rapidly forming. In fact, this tiny organ will begin to pump blood and divide into chambers in week five of your pregnancy. Muscles, bone and tissue will be products of this layer.
By this point, the endoderm has also formed and is in the beginning stages of producing the lungs, intestines, pancreas, thyroid and other such vital organs.
So, if you're extra tired this week, give yourself a break. You've been developing a central nervous system, for crying out loud! You're entitled to an extra nap or two, even if your child more closely resembles a tadpole now than the infant you can't wait to snuggle.