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.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 (called the "anatomy scan", "anomaly scan," or "level 2 ultrasound").Some resources indicate that there are clear reasons for this and that such scans are also clearly beneficial because ultrasound enables clear clinical advantages for assessing the developing fetus in terms of morphology, bone shape, skeletal features, fetal heart function, volume evaluation, fetal lung maturity, Second-trimester ultrasound screening for aneuploi- dies is based on looking for soft markers and some predefined structural abnormalities.
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Performing an ultrasound at this early stage of pregnancy can more accurately confirm the timing of the pregnancy and can also assess for multiple fetuses and major congenital abnormalities at an earlier stage.Not useful for dating, the abdominal circumference of the fetus may also be measured.This gives an estimate of the weight and size of the fetus and is important when doing serial ultrasounds to monitor fetal growth.In most countries, routine pregnancy sonographic scans are performed to detect developmental defects before birth.
This includes checking the status of the limbs and vital organs, as well as (sometimes) specific tests for abnormalities.
In general, the earliest scans are more accurate than dating scans performed later in pregnancy…