Is Down syndrome screening for twins accurate?

Being pregnant with twins is something that many mothers are very happy about, because they can have two babies at once. However, for some families with a history of genetic diseases or the possibility of recessive inheritance, they are very worried that the baby will have some genetic diseases, such as gene mutation or genetically caused Down's syndrome. At this time, they need to do Down's syndrome screening. So what is the process of Down's syndrome examination and is it effective?

Basic Overview

Down syndrome, also known as trisomy 21, means that the patient's 21st pair of chromosomes has one more than the normal person. The normal person has one pair, which is the most common chromosomal aneuploidy disease. Down syndrome screening is performed at a specific gestational week by testing the levels of PAPPA, AFP, HCG, uE3 and Inhibin A in the pregnant woman's serum, combined with clinical information such as the pregnant woman's age, gestational age, weight, whether she smokes and has insulin-dependent diabetes, and calculating the risk value through risk assessment software.

The critical value is 1/250-380 and may vary due to different methodologies. Greater than is high risk and less than is low risk. The probability of having Down syndrome DS in the general population under 35 years old is 1/750. According to the examination time, it is divided into early pregnancy 9-13 weeks and mid-pregnancy 14-21 weeks.

Down syndrome screening test is the abbreviation of prenatal screening test for Down syndrome. The purpose is to comprehensively judge the risk of the fetus suffering from Down syndrome by testing the pregnant woman's blood in combination with other clinical information. If the Down screening test results show that the fetus has a higher risk of Down syndrome, further confirmatory tests should be carried out - amniocentesis or chorionic villus sampling.

Screening process

First trimester

This examination mainly uses two screening methods, "ultrasound" and "blood test", when the pregnant mother is between 10 and 13+6 weeks of pregnancy. Dr. Huang Xuguang said that ultrasound can clearly measure the thickness of the fetal nuchal translucency, and blood tests can be used to measure the plasma protein A value and human chorionic gonadotropin value in the mother's serum to estimate the risk of the fetus suffering from Down syndrome.

Second Trimester

When pregnant mothers have their blood drawn at 15 to 20 weeks, the hospital will test the serum levels of alpha-fetoprotein, human chorionic gonadotropin, free estriol and inhibin, and then calculate the risk of the fetus suffering from Down syndrome based on the mother's age, weeks of pregnancy and weight.

Chorionic villus sampling

Dr. Huang Xuguang said that pregnant mothers must discuss this test carefully with their doctors first because it is an invasive test that requires obtaining some cell samples from the developing placenta. He gave an example that when a pregnant mother undergoes first trimester screening, if the fetal nuchal translucency is more than 3 mm, she can consider doing chorionic villus sampling directly to test the chromosomes. However, the results of this will not be known until 14 days later.

Amniocentesis

This is also an invasive test but carries fewer risks than chorionic villus sampling. Dr. Huang Xuguang said that by extracting amniotic fluid from the amniotic cavity of the pregnant mother's uterus for testing, it can be determined whether the fetus's chromosomes are abnormal and whether the fetus is likely to have Down syndrome. It takes 14 days to get the results and its accuracy is over 99%.