Fetal Development--First Trimester

Although the fetus weighs well under a pound, the major organs and systems of the body are formed by the end of the first trimester.


The embryo develops a simple brain, spine and central nervous system.

Weeks 3-8: Greatest sensitivity to teratogens.

Week 4: The skin cells begin to form in two layers.


Week 5: Fetal cells in mother's blood can be used to identify birth defects.

The heart is less than a half-inch in diameter and begins to beat.

Spots appear where eyes will form; face is almost recognizable.


Week 10: Chorionic villus sampling to test for birth defects can be performed.

Fetus is size of tennis ball.

Eyelids close over eyes and vocal cords form.

Pulses of thyroid hormones begin to regulate brain development.

By the end of 12th week, the mass of the fertilized egg has increased 2.5 million times. During the rest of pregnancy, the increase is only 230-fold.

Blood pressure screening can identify women at high risk for preeclampsia.

First-Trimester Glossary:

Teratogens: Substances or agents that are known to be dangerous to the fetus.

Sebaceous: Oil glands of the skin.

Chorionic villus sampling: An invasive technique in which a needle is inserted through the mother's abdomen to remove a tiny piece of the placenta. The tissue is analyzed for genetic defects.

Preeclampsia: A serious disease of late pregnancy (cause unknown) that leads to high blood pressure and increased fluids. Can advance to eclampsia if disease is not brought under control. In severe cases, the mother can have seizures.

Sources: "Reproductive Health," American Heart Assn., March of Dimes, Anatomy and Physiology the Easy Way; Researched by Thomas Maugh II

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