During the fetal stage, the body grows larger, the proportions of the features are refined, and organ development is completed. During the seventh and eighth weeks, the body grows more erect, the chest area develops, and the face begins to acquire a human look. In the third month, facial features continue to develop, nails form, ossification centers develop in bones, the sex of the unborn can be determined, and the fetus is capable of responding to outside stimulation. During the second trimester (fourth to sixth months), distinctive facial features develop, the fetal heartbeat can be detected, and fetal quickening (movements) can be felt externally. In the third trimester (the seventh to ninth months), the body proportions, except for the somewhat large head, are established, the skin becomes smoother, and the organs develop sufficiently for the newborn to function on its own.
If the fetus is expelled before 36 weeks of gestation are completed, it often can survive outside the womb, but artificial assistance, such as intravenous feedings and strict maintainance of the ambient temperature, may be needed during the remainder of its normal developmental period. Such births are called premature. Fetuses expelled before that period are not viable and are termed either a miscarriage or an abortion. A dead fetus delivered in the third trimester is termed stillborn.
See also reproductive system.
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