Fetal Development Weeks 5 Through 8

See what's happening with your baby in the second month of your pregnancy.

Week 5

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.

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