Answer:

Down syndrome (Trisomy 21) is the most common genetic abnormality, affecting one in 700 live births. It occurs when an individual has three copies of chromosome (NST) 21 instead of the usual two. About 95% of these abnormalities arise randomly during embryo formation and rarely recur.

The fact that your brother has two children with Down syndrome suggests that he and his wife may carry chromosomal translocations, meaning an extra part of chromosome 21 is attached to another chromosome (possibly chromosome 13, 14, 15, or 22). Individuals carrying such a translocation are completely healthy because their genetic material is neither excessive nor deficient; this is known as a balanced translocation. However, during egg or sperm formation, this chromosome can segregate unevenly, creating abnormal gametes, which leads to a high risk of the fetus having Down syndrome.

To accurately determine the cause, your brother's children need a blood test to check if they have standard trisomy 21 Down syndrome (three separate chromosome 21s) or translocation Down syndrome (an extra copy of chromosome 21 attached to another chromosome). From there, doctors can advise you to undergo a karyotype analysis to examine your chromosomes for any balanced translocations involving chromosome 21, thereby assessing your risk of having a child with Down syndrome.

Dr. Thao provides genetic counseling. *Illustration: Tam Anh District 7 General Clinic*

If you carry a chromosome 21 translocation, you can undergo non-invasive prenatal testing (NIPT) from gestational week 9 and a morphological ultrasound at week 12 to screen for the risk of the fetus having Down syndrome. If the results indicate a high risk, doctors can perform chorionic villus sampling or amniocentesis to accurately determine if the fetus has the syndrome. Depending on the family's wishes, doctors will develop an appropriate pregnancy management plan. You may also opt for in vitro fertilization (IVF) with preimplantation genetic testing (PGT) to screen out embryos with chromosomal abnormalities.

Dr. Nguyen Phuong Thao

Specialty: Fetal Medicine

Tam Anh District 7 General Clinic