From the Guidelines
A 1-week embryo with mosaicism, specifically 50% XY and 50% -22, has a low likelihood of developing into a normal baby and having a normal pregnancy, due to the high risk of early miscarriage and potential for various degrees of abnormalities. The presence of mosaicism, particularly with a significant percentage of affected cells, can affect fetal development and increase the risk of adverse pregnancy outcomes 1. According to the American College of Medical Genetics and Genomics (ACMG), mosaicism identified at the time of chorionic villi sampling (CVS) occurs in 1% to 2% of pregnancies, and the large majority represent confined placental mosaicism (CPM) 1.
The incidence of mosaic RATs identified at the time of CVS is 0.6%, and only 2.53% of these cases were confirmed through amniocentesis, highlighting the complexity of interpreting these results 1. The ACMG recommends follow-up amniocentesis to clarify the status of the fetus with respect to the mosaicism detected on CVS, and consultation with a genetics healthcare professional is prudent when considering noninvasive prenatal screening (NIPS) for CNVs 1.
Given the limited understanding of the clinical implications of mosaicism and the potential for variable outcomes, consultation with a maternal-fetal medicine specialist and genetic counselor is essential to understand the specific implications of this chromosomal finding and to discuss the potential risks and benefits of continuing the pregnancy. The use of NIPS for CNVs is not routinely recommended due to insufficient evidence and undetermined clinical utility, but it may be considered on a case-by-case basis, taking into account the pregnancy and family history 1.
Key considerations in the management of this pregnancy include:
- The type and percentage of mosaicism present
- The potential for confined placental mosaicism (CPM)
- The risk of early miscarriage and adverse pregnancy outcomes
- The need for follow-up amniocentesis and genetic counseling
- The potential benefits and limitations of NIPS for CNVs in this context 1.
From the Research
Mosaicism and Pregnancy Outcomes
- Mosaicism, specifically 50% XY and 50% -22, refers to the presence of two or more different cell populations with distinct genetic makeup in an individual 2.
- The development of a normal baby with this condition is uncertain, as it depends on the proportion and distribution of abnormal cells in the fetus.
- Studies have shown that mosaicism can be confined to the placenta or present in both the placenta and the fetus, which can affect pregnancy outcomes 3, 4.
Detection and Diagnosis
- Non-invasive prenatal testing (NIPT) can detect chromosomal abnormalities, including mosaicism, but may not always provide a definitive result 5, 6.
- Invasive testing, such as chorionic villus sampling (CVS) or amniocentesis, may be necessary to confirm a positive NIPT result and determine the extent of mosaicism 2, 3, 4.
- The choice between CVS and amniocentesis depends on various factors, including gestational age, ultrasound findings, and the type of chromosomal abnormality suspected 4.
Pregnancy Outcomes with Mosaicism
- The presence of mosaicism can increase the risk of pregnancy complications, such as fetal growth restriction, oligohydramnios, and miscarriage 2.
- However, some studies have reported cases of normal pregnancy outcomes despite the presence of mosaicism, highlighting the importance of individualized management and monitoring 6.
- Further research is needed to understand the relationship between mosaicism and pregnancy outcomes, as well as to develop more accurate and reliable diagnostic methods 5, 3.