NIPT, or non-invasive prenatal screening, is a test that helps detect the early risk of fetal abnormalities. It works by analyzing cell-free placental DNA circulating in the mother's blood.

According to the American College of Obstetricians and Gynecologists (ACOG) and the American College of Medical Genetics and Genomics (ACMG), NIPT is the preferred screening option for twin pregnancies. This method is more effective than maternal serum biochemical tests, such as the double test and triple test, in detecting common chromosomal abnormalities like Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13). The effectiveness in screening for Down syndrome reaches 98%, with a very low false positive rate.

However, NIPT in twin pregnancies has certain limitations due to the DNA concentrations of both fetuses mixing in the mother's blood. The rate of inconclusive results in twin pregnancies is higher than in singleton pregnancies. If a high-risk result is obtained, the test identifies a general abnormality but does not pinpoint which fetus is affected. Furthermore, screening for microdeletions or sex chromosome aneuploidies often has low reliability and is not recommended for twin pregnancies.

The accuracy and complexity of this technique for twin pregnancies depend on several factors. If it is a monozygotic (identical) twin pregnancy, the analysis effectiveness is nearly as high as for a singleton pregnancy. Conversely, dizygotic (fraternal) twin pregnancies or those resulting from in vitro fertilization (IVF) make the DNA signal more complex, which can lead to inaccurate results. The vanishing twin phenomenon, where one fetus stops developing early, can also affect the accuracy for the remaining fetus.

Compared to singleton pregnancies, the overall effectiveness of NIPT in twin pregnancies is slightly reduced, especially for screening for Edwards syndrome and Patau syndrome. Therefore, if a high-risk result is received, pregnant women may be advised to undergo diagnostic testing via amniocentesis.

A pregnant mother undergoes blood sampling for prenatal screening. Illustration: Tam Anh General Clinic District 7

At 12 weeks of gestation, it is recommended to visit a specialized medical center. Doctors can perform an ultrasound to measure the nuchal translucency of both fetuses, combined with NIPT, to provide the most comprehensive risk assessment. NIPT cannot replace fetal ultrasound. While NIPT screens for genetic abnormalities, ultrasound is an essential tool for detecting morphological and structural fetal anomalies at an early stage.

Pregnant women carrying twins should be monitored at each stage of pregnancy, based on the type of twin pregnancy and the specific risks of each case. Doctors do not solely rely on NIPT results but integrate maternal age, pregnancy history, fetal characteristics, and ultrasound findings to assess risk. If screening results indicate high risk or ultrasound reveals abnormalities, the pregnant woman receives in-depth counseling and is prescribed appropriate diagnostic tests before making further decisions.

Dr. Nguyen Phuong Thao, Master of Science

Fetal Medicine Unit

Tam Anh General Clinic District 7