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Gestational age is usually determined by the date of the woman's last menstrual period, and assuming ovulation occurred on day fourteen of the menstrual cycle.
Sometimes a woman may be uncertain of the date of her last menstrual period, or there may be reason to suspect ovulation occurred significantly earlier or later than the fourteenth day of her cycle.
Ultrasound scans offer an alternative method of estimating gestational age.
The most accurate measurement for dating is the crown-rump length of the fetus, which can be done between 7 and 13 weeks of gestation.
In normal state, each body tissue type, such as liver, spleen or kidney, has a unique echogenicity.
Fortunately, gestational sac, yolk sac and embryo are surrounded by hyperechoic (brighter) body tissues.
Research shows that routine obstetric ultrasound before 24 weeks gestational age can significantly reduce the risk of failing to recognize multiple gestations and can improve pregnancy dating to reduce the risk of labor induction for post-dates pregnancy.
The procedure is a standard part of prenatal care in many countries, as it can provide a variety of information about the health of the mother, the timing and progress of the pregnancy, and the health and development of the embryo or fetus.Although 91% of fetuses affected by Down syndrome exhibit this defect, 5% of fetuses flagged by the test do not have Down syndrome. Usually scans for this type of detection are done around 18 to 23 weeks of gestational age.