Trisomy 2 mosaicism is a rare chromosome disorder characterized by having an extra copy of chromosome 2 in a proportion, but not all, of a person’s cells. Many cases of trisomy 2 mosaicism result in miscarriage during pregnancy. In infants born with trisomy 2 mosaicism, severity as well as signs and symptoms vary widely. Features of trisomy 2 mosaicism may include intrauterine growth restriction (IUGR), any of various birth defects, distinctive facial features, growth delay, developmental delays, and intellectual disabilities. However, children with trisomy 2 mosaicism with no significant medical problems have been reported (although long-term follow-up was not available). The severity and specific symptoms present generally depend on the level of mosaicism (the proportion of cells affected) and the location or type of affected cells in the body. Trisomy 2 mosaicism is not inherited. It is caused by a random error in cell division during early development of the embryo.

When trisomy 2 mosaicism is detected during early pregnancy with chorionic villus sampling (CVS), the affected cells may be confined only to the placenta, and not present in the fetus. Amniocentesis is typically recommended to confirm this, and monitoring is still warranted due to an increased risk for intrauterine growth restriction, low amniotic fluid level (oligohydramnios), or other complications including stillbirth.

During pregnancy (prenatally), trisomy 2 mosaicism may be associated with various findings, such as:

• elevated maternal serum alpha-fetoprotein (MSAFP)
• abnormal results of other maternal serum screening tests
• ultrasound findings such as intrauterine growth restriction; low amniotic fluid level (oligohydramnios); congenital heart defects; enlargement of fluid-filled structures in the brain (ventriculomegaly); spina bifida; and swelling of the kidneys (hydronephrosis)

Signs and symptoms that have been reported in livebirths with trisomy 2 mosaicism include:

• distinctive head and facial features such as a flattened appearance of the middle part of the face (midface hypoplasia), absence of one or both eyes (anophthalmia) or abnormally small eyes (microphthalmia), cleft lip and palate, wide-set eyes (hypertelorism), and small head size (microcephaly)
• growth and motor delay
• intellectual disability
• congenital heart defects
• neural tube defects
• diaphragmatic hernia
• inguinal hernia
• radioulnar hypoplasia
• rocker-bottom feet (feet with a rounded bottom, resembling the bottom of a rocking chair)
• abnormal development of the lower end of the spine (caudal dysgenesis)
• portal fibrosis
• intestinal malrotation (twisting of the intestines)
• Hirschsprung disease
• hypomelanosis of Ito
• polydactyly
• deafness
• undescended testes
• face and body asymmetry
• clubfoot
• lack of sacrum

Because few cases have been reported in the literature and the level of mosaicism differs among affected fetuses and individuals, it is not possible to predict how a pregnancy or person may be affected by trisomy 2 mosaicism.

When trisomy 2 mosaicism is diagnosed (in a fetus or child), it can be difficult to estimate the true level of mosaicism because levels can differ depending on the nature of the sample tested. Different levels of mosaicism may be found in the same pregnancy using various lab techniques in samples obtained by chorionic villus sampling, amniocentesis (both uncultured and cultured cells), and cord blood sampling. Likewise, different levels of mosaicism may be found in a liveborn when samples are obtained for urinary analysis, blood analysis, or tissue analysis. For example, a blood test can only determine the level of mosaicism in the blood cells.

During pregnancy, because it is not possible to predict the outcome for all fetuses diagnosed with trisomy 2 mosaicism, health care professionals may recommend a variety of additional tests to obtain as much information as possible about how a fetus is developing. Tests that may be recommended may include analysis of other types of prenatal samples, more detailed ultrasound exams, fetal echocardiography to evaluate the fetal heart, and/or fetal MRI. Prognosis information in each individual case may be based on results of these tests and the outlook associated with specific findings.