How A Fetus Grows

How A Fetus Develops in the Third Trimester

The fetus is fully developed by the beginning of the third trimester. The last three months, therefore, are devoted to growth. By the seventh month, the fetus is about 14 to 17 inches long and weighs about two pounds. The skin is red, wrinkled, and thin. It becomes covered with vernix, a thick, white, sticky material composed of skin cells, lanugo, and oily skin secretions.

A baby born at this time has at least a 60 percent chance of survival with appropriate medical care, and the survival rate increases with each passing day as the baby develops. Babies born this early can respond to taste, light, and sound.

The chance of survival for a baby born during the eighth month of gestation increases to 90 percent. By this time, the fetus is 16 to 18 inches long and weighs three to four pounds.

The final preparations for independent existence occur during the ninth month. Surfactant, a substance that lines the lungs and allows them to expand easily, develops. Fat is stored, and its deposition under the skin smoothes out the wrinkles. Much of the lanugo disappears.

By the final month of pregnancy, the fetus is usually 20 to 22 inches long and weighs seven to eight pounds. The baby is large and strong enough for the next step -- birth and independent life. That one cell has come a long way, from embryo to fetus to newborn.

Though it is only nine months, some new mothers feel that their pregnancy will last forever. During this time of both slow and rapid change, it's important to have a sense of the complete process to understand what is happening to you and your baby.


Dr. Elizabeth Eden, M.D. is a practicing obstetrician with her own private practice in New York City. She serves as an attending physician at the Tisch Hospital of the New York University Medical Center, as well as a Clinical Assistant Professor at the New York University School of Medicine.

This information is solely for informational purposes. IT IS NOT INTENDED TO PROVIDE MEDICAL ADVICE. Neither the Editors of Consumer Guide (R), Publications International, Ltd., the author nor publisher take responsibility for any possible consequences from any treatment, procedure, exercise, dietary modification, action or application of medication which results from reading or following the information contained in this information. The publication of this information does not constitute the practice of medicine, and this information does not replace the advice of your physician or other health care provider. Before undertaking any course of treatment, the reader must seek the advice of their physician or other health care provider.


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