Obstetric sonography (ultrasonography) is the application of medical ultrasonography to obstetrics. Here, ultrasound is used to visualize the embryo or fetus in its mother's uterus (womb). In some countries, routine pregnancy ultrasound scans are performed to detect developmental defects before birth, the perhaps most common method uses a measurement of the nuchal translucency thickness ("NT-test", or "Nuchal Scan") which can predict with high accuracy whether the fetus is affected by the Down syndrome.
The sex of the baby can usually be determined any time after 16 weeks, often at the dating scan around 20 weeks into the pregnancy depending upon the quality of the sonographic machine and skill of the operator. This is also the best time to have an ultrasound done as most infants are the same size at this stage of development. Depending on the skill of the sonographer, ultrasound may suffer from a high rate of false negatives and false positives, that means care has to be taken in interpreting the accuracy of the scan.
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