Amniocentesis is the removal of the amniotic fluid from the amniotic cavity. As the fetus develops, molecules of various types, as well as living cells are expelled into the amniotic fluid. These molecules and cells can be collected and analyzed. A number of normal conditions can be evaluated, and a number of metabolic disorders can be detected by analysis of the types of molecules expelled by the fetus. The cells collected by amniocentesis can be grown in culture, and additional metabolic disorders can be evaluated. Chromosome analysis, call a karyotype, can also be accomplished on the cultured cells. Amniocentesis has been performed as early as 10 weeks after fertilization, but the success rate at that time is quite low. It is most commonly performed at 13 to 14 weeks after fertilization.

Fetal tissue samples may also be obtained by chorionic villi sampling, in which a probe is introduced into the uterine cavity through the cervix and a small piece of chorion is removed. This technique has an advantage over amniocentesis in that it can be used earlier in development, as early as the seventh to ninth week after fertilization.

One of the molecules normally produced by the fetus and released into the amniotic fluid is alpha-fetoprotein. If the fetus has tissues exposed to the amniotic fluid that are normally covered by skin, such as nervous tissue, resulting from failure of the neutral tube to close, or abdominal tissues, resulting from failure of the abdominal wall to fully form, an excessive amount of alpha-fetoprotein will be lost into the amniotic fluid.

Some of the metabolic by-products from the fetus, such as alpha-fetoprotein and estriol, which is a weak form of estrogen, produced in the placenta after 20 weeks of gestation, can enter the maternal blood and in some cases can be processed and passed to the maternal urine. The levels of these fetal products can then be measured in the mother's blood or urine.

The fetus can be seen within the uterus by ultrasound, which uses sound waves that are bounced off the fetus like sonar and then analyzed and enhanced by computer; or by fetoscopy, in which a fiberoptic probe is introduced into the amniotic cavity. Because of the constantly increasing resolution in ultrasound and because it is noninvasive compared to fetoscopy, the latter technique is not commonly used at present. The technique of ultrasound has not been found to pose any risk to the fetus or mother. Ultrasound can be accomplished by placing a transducer on the abdominal wall (transabdominal) or by inserting the transducer into the woman's vagina (transvaginal). The latter technique produces much higher resolution because there are less layers of tissue between the transducer and the uterine cavity. Transvaginal ultrasound can be used to identify the yoke sac of a developing embryo as early as 17 days after fertilization, and the embryo can be visualized by 25 days. Transabdominal ultrasound allows for fetal monitoring by 6 to 8 weeks after fertilization.

Fetal heart rate can be detected with an ultrasound stethoscope by the 10 week after fertilization and with a conventional stethoscope by 20 weeks. The normal fetal heart rate is 140 beats per minute (normal range 110 to 160).

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