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False negative readings can occur when testing is done too early. hCG levels rise rapidly in early pregnancy and the chances of false negative test results diminish with time (increasing gestational age). [24] Less sensitive urine tests and qualitative blood tests may not detect pregnancy until three or four days after implantation. [25]
The results of the blood test are then combined with the NT ultrasound measurements, maternal age, and gestational age of the fetus to yield a risk score for Down syndrome, trisomy 18, and trisomy 13. First Trimester Combined Test has a sensitivity (i.e. detection rate for abnormalities) of 82–87% and a false-positive rate of around 5%. [73] [74]
The CST is used for its high negative predictive value. A negative result is highly predictive of fetal wellbeing and tolerance of labor. The test has a poor positive predictive value with false-positive results in as many as 30% of cases. [4] [5] A positive CST indicates high risk of fetal death due to hypoxia [3] and is a contraindication to ...
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If you get two negative at-home COVID test results 48 hours apart after previously testing positive, you are likely no longer contagious. But how long that will take is "wholly dependent on the ...
The sex of the fetus may be discerned by ultrasound as early as 11 weeks' gestation. The accuracy is relatively imprecise when attempted early. [15] [16] [17] After 13 weeks' gestation, a high accuracy of between 99% and 100% is possible if the fetus does not display intersex external characteristics. [18]
In another study values of 79.6% and 2.7% for the combined screening were then improved with the addition of second trimester ultrasound scanning to 89.7% and 4.2% respectively. [13] A further study reported detection of 88% for trisomy 21 (Down syndrome) and 75% for trisomy 18 ( Edwards syndrome ), with a 3.3% false-positive rate. [ 14 ]