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embryology.pep.zone

THE FETAL PERIOD

* The fetal period begins 9 weeks after fertilization (11 weeks after the LNMP) and ends at birth. It is characterized by rapid body growth and differentiation of tissues and organ systems. An obvious change in the fetal period is the relative slowing of head growth compared with that of the rest of the body.
* By the beginning of the 20th week, lanugo and head hair appear, and the skin is coated with vernix caseosa. The eyelids are closed during most of the fetal period but begin to reopen at approximately 26 weeks. At this time, the fetus is usually capable of extrauterine existence, mainly because of the maturity of its respiratory system.
* Until approximately 30 weeks, the fetus appears reddish and wizened because of the thinness of its skin and the relative absence of subcutaneous fat. Fat usually develops rapidly during the last 6 to 8 weeks, giving the fetus a smooth, plump appearance.
* The fetus is less vulnerable to the teratogenic effects of drugs, viruses, and radiation, but these agents may interfere with growth and normal functional development, especially of the brain and eyes.
* The physician can now determine whether a fetus has a particular disease or a congenital anomaly by using various diagnostic techniques, e.g., amniocentesis, CVS, ultrasonography, and magnetic resonance imaging. Prenatal diagnosis can be made early enough to allow early termination of the pregnancy if elected, e.g., when severe anomalies incompatible with postnatal life are diagnosed, such as absence of most of the brain.
* In selected cases, treatments can be given to the fetus, e.g., the administration of drugs to correct cardiac arrhythmia or thyroid disorders. Surgical correction of some congenital anomalies in utero (Fig. 6-18) is also possible (e.g., fetuses that have ureters that do not open into the bladder).




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