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Fetal Development Weeks 5 Through 8

Congratulations! The date your period would have normally started has come and gone, and by now you've probably taken a pregnancy test and learned the big news. That positive urine-test result was caused by your body's raised level of the telltale "pregnancy hormone," human chorionic gonadotropin (hCG). This is the magical stuff that makes your period vanish for nine months.

You've only just found out that a baby is on the way, but deep inside the uterus, the first traces of the fetus's body are already beginning to appear. A "primitive streak" forms on the surface of the embryonic cell cluster: this pair of ridges will join to form the spinal cord and the brain. Folic acid helps this early development get started in just the right way -- and that's why it's incredibly important to get lots of this vital nutrient into your system as early as possible.

About sixteen days after ovulation, the embryo's cells organize themselves into three specialized layers: the ectoderm, mesoderm, and endoderm. These layers eventually give rise to all of baby's tiny parts:

  • Endoderm forms the lungs, tongue, tonsils, urethra and associated glands, bladder and digestive tract.
  • Mesoderm forms the muscles, bones, lymphatic tissue, spleen, blood cells, heart, and reproductive and excretory systems.
  • Ectoderm forms the skin, nails, hair, lens of eye, lining of the internal and external ear, nose, sinuses, mouth, anus, tooth enamel, pituitary gland, mammary glands, and all parts of the nervous system.

Hard to believe: though he's no bigger than a lentil, your baby already has a beating heart! By the middle of this week, his cardiac muscles have formed and begun fluttery little flexing movements. This mini -version of the human heart is your baby's first working organ.

For now, this little being still looks more like a curled-up tadpole than a human baby. A huge head makes up almost half the embryo's length (the better to accommodate a rapidly growing brain). In the midsection, near the bulge that holds the heart, tiny arm and leg buds have appeared. Just below them is the attachment point of the umbilical cord, slender as a thread. The cord connects the fetus to the growing placenta, the life-giving organ that has formed to supply the growing baby with the blood supply and nutrients needed for survival.

The body ends in a long tail. But don't worry--this reminder of man's evolutionary roots disappears within a few weeks, absorbed into the spinal cord.

Inside the body, the central nervous system, the circulatory system, and organs like the liver and pancreas are developing. Along the embryo's "back" is a row of small bony blocks called somites. These will become the spine and ribs, among other things.

By the end of this week, your baby measures about one-third of an inch from crown to rump. (Doctors use this "sitting height" to track early growth because baby's legs are hard to measure). Curled up, she's about as big as a pea. But she's a pea with personality: her facial features are starting to emerge -- small hollows that will become eyes, inner ears, and the beginnings of mouth and tongue.

Around the embryo's tiny body, a first layer of skin forms, delicate and translucent. The ends of the arm buds flatten into little paddles called "hand plates." By the end of next week, fingers will have formed. In this early stage, they are webbed, like duck's feet.

Amazing changes are taking place in the embryo's brain right now. Its cells are dividing into three sections. First is the forebrain, home base to the human personality. This is where problems will be solved and the avalanche of information gathered by the senses -- sounds, smells, tastes -- will be processed. Next comes the midbrain, which will serve as a relay station, coordinating messages and sending them on to their final destinations in the body. Last is the hindbrain, responsible for regulating the heart, breathing, and muscle movements. All three parts, working together, form the miraculous command center that is the human brain.

Your baby is growing -- he's now a tiny bundle about the size of a lima bean. He is well-cushioned in his uterine home, afloat in the clear "waters" of the ever-growing amniotic sac. Here, he can drift around freely, while still remaining attached to the placenta by the bungee-like umbilical cord. The placenta is busy processing nutrients from your bloodstream to nourish the embryo, while also disposing of waste produced by him.

At this point, the embryo's entire body is beginning to uncurl a bit, but its head is still very large in proportion to the rest of it. This will also be true after birth: a newborn's head makes up about a quarter of its length, while an adult's is only an eighth.

The esophagus, which brings food to the stomach, forms this week, and on either side of it, two buds that are the beginning of lungs. Imagine: it will be eight months until those lungs gulp their first breath of air.

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Reviewed 2008

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