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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.
Soft markers are variations from normal anatomy, which are more common in aneuploid fetuses compared to euploid ones.
A gestational sac can be reliably seen on transvaginal ultrasound by 5 weeks' gestational age (approximately 3 weeks after ovulation).
Additionally, the ISUOG recommends that pregnant women have obstetric ultrasounds between 11 weeks' and 13 weeks 6 days' gestational age in countries with resources to perform them (the nucal scan).
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.
Dating is more accurate when done earlier in the pregnancy; if a later scan gives a different estimate of gestational age, the estimated age is not normally changed but rather it is assumed the fetus is not growing at the expected rate.
Not useful for dating, the abdominal circumference of the fetus may also be measured.
Fortunately, gestational sac, yolk sac and embryo are surrounded by hyperechoic (brighter) body tissues.