The best time for mid-term screening is between 16 and 18 weeks of pregnancy, which is mainly a blood test. In fact, the accuracy rate of the screening test itself is about 60 percent. If the results of the early screening test can be considered together with the results of the mid-term screening test, the accuracy rate can be improved. The accuracy rate is about 70 percent, while the mid-term screening is mainly a blood test for HCG, alpha-fetoprotein and estriol, combined with the age and weight of the pregnant woman to consider whether the probability of having Down’s syndrome is high risk or low risk, in fact, even low risk cannot be 100% excluded from Down’s syndrome, and high risk does not mean that it will definitely be Down’s syndrome. The probability of having Down’s syndrome is only the probability of it happening. The reason is that many high-risk cases are eventually normal when further non-invasive genetic testing is done. In conclusion, early and mid-term screening are both tests to detect Down’s syndrome, but the timing of these tests is different. The former is done mainly by ultrasound, usually around 11 to 12 weeks of pregnancy, while the latter is done mainly by blood sampling, usually between 16 and 18 weeks of pregnancy.
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