My patient’s NIPT is positive for monosomy X (Turner syndrome). What does this mean?
Your patient’s NIPT result suggests the presence of one X sex chromosome and the absence of a second sex chromosome. NIPT is a screening test; false positives, though rare, can occur. The actual chance for the pregnancy to have monosomy X depends on many factors, including the patient’s clinical and family history.
Next steps to consider: You should discuss the results and the potential clinical implications with your patient. The American College of Obstetricians and Gynecologists and the Society for Maternal-Fetal Medicine state, “All women with a positive cell-free DNA test result should have further detailed counseling and testing and should have a diagnostic procedure before any irreversible action is taken.” Confirmation prior to birth can also help with pregnancy and neonatal management.
What is Monosomy X?
Monosomy X is a condition that is caused by having one X sex chromosome and an absent second sex chromosome (one X sex chromosome instead of two sex chromosomes).
What are the features of Monosomy X?
Many pregnancies with monosomy X will result in a pregnancy loss; however, monosomy X is compatible with continued survival and live birth. Females with monosomy X have variable phenotypes. Typically, females with monosomy X have normal intelligence; however, learning disabilities are possible and variable. Some of the common features of monosomy X include heart defects, kidney abnormalities, short stature, congenital lymphedema, and primary amenorrhea.
What is the prevalence of this condition?
Approximately 1 in 2500 females are born with monosomy X. This condition usually happens by chance and is not typically associated with advanced parental age.
What testing should be considered?
- Specialized genetic tests such as karyotyping, fluorescence in situ hybridization (FISH), qPCR, and microarray are available to confirm the presence of monosomy X.
- These confirmatory tests are generally performed on cells from chorionic villus sampling (CVS) or amniocentesis during pregnancy, on cord blood or peripheral blood sample after the baby is born, or on products of conception (POC) in the case of miscarriage.
- Ultrasound evaluation may be useful in aiding with a prenatal diagnosis of monosomy X, but a normal ultrasound cannot exclude this condition.