Trisomy 18, also known as Edwards syndrome, is a severe genetic condition that affects how a baby develops and grows. It is a chromosomal disorder, meaning it involves an abnormality in the number of chromosomes. Prenatal screening, particularly through ultrasound, plays a role in identifying potential indicators of this condition during pregnancy.
Understanding Trisomy 18
Trisomy 18 occurs when an individual has an extra copy of chromosome 18, instead of the usual two. This additional genetic material disrupts normal development, leading to various physical differences and health challenges. The condition is often characterized by slow growth during fetal development, low birth weight, and multiple birth differences. These can include a small head, a small jaw, and often, clenched fists with overlapping fingers. Most cases of Trisomy 18 happen randomly and are not inherited.
Ultrasound’s Role in Early Detection
Ultrasound serves as a screening tool during pregnancy to identify potential risks for chromosomal conditions like Trisomy 18. This imaging technique can provide early indications, particularly within the first trimester. One common early marker is an increased nuchal translucency (NT), which is a measurement of the fluid-filled space at the back of the baby’s neck. An elevated NT measurement, typically taken between 11 and 14 weeks of gestation, can suggest an increased risk for Trisomy 18 and other chromosomal abnormalities.
Another early indicator that may be observed is the absence or hypoplasia (underdevelopment) of the fetal nasal bone. While an absent nasal bone can sometimes occur in normally developing fetuses, it is more frequently observed in cases of chromosomal conditions, including Trisomy 18. These first-trimester ultrasound findings are considered screening tools, not definitive diagnoses. They indicate a higher chance of a chromosomal abnormality, prompting further discussion and potential diagnostic testing.
Specific Ultrasound Markers and Timelines
Specific ultrasound markers associated with Trisomy 18 can be observed at various stages of pregnancy. Some are potentially visible as early as the first trimester, while others become more apparent during the second-trimester anatomy scan. In the first trimester, between 11 and 14 weeks, an increased nuchal translucency is a notable finding, observed in a high percentage of Trisomy 18 cases. An absent or hypoplastic nasal bone can also be detected during this period in approximately 50-53% of fetuses with Trisomy 18. Additionally, early-onset fetal growth restriction and omphalocele, where abdominal organs protrude outside the belly, might be seen. Some cardiac anomalies may also be detectable through specialized echocardiography.
More comprehensive assessment of specific markers typically occurs during the detailed second-trimester anatomy scan, usually performed between 18 and 22 weeks of gestation. At this stage, a range of structural differences can often be identified.
Common findings include clenched fists with overlapping fingers, which are a characteristic feature of Trisomy 18. Another skeletal marker is “rocker-bottom feet,” where the sole of the foot has a convex, rounded appearance.
Other frequently observed anomalies in the second trimester include:
Choroid plexus cysts, small fluid-filled sacs in the brain, found in about one-third of fetuses with Trisomy 18. While isolated cysts often resolve, their presence, especially when large or bilateral or combined with other markers, can raise concern.
Congenital heart defects, affecting nearly all cases, with ventricular septal defects (VSD) and atrial septal defects (ASD) being frequently identified.
Omphalocele.
Kidney abnormalities, such as polycystic kidneys or hydronephrosis.
Intrauterine growth restriction, which often becomes more pronounced.
Confirming a Suspected Diagnosis
When ultrasound findings suggest an increased risk for Trisomy 18, further diagnostic tests are typically recommended to confirm the diagnosis. Non-invasive prenatal testing (NIPT) is often the next step in the screening process. NIPT involves a maternal blood test that analyzes fragments of fetal DNA circulating in the mother’s bloodstream, providing a highly accurate risk assessment for conditions like Trisomy 18.
For a definitive diagnosis, invasive procedures are necessary. Chorionic villus sampling (CVS) can be performed earlier in pregnancy, typically between 10 and 13 weeks of gestation, by taking a small tissue sample from the placenta. Amniocentesis, performed usually after 15 weeks, involves collecting a small amount of amniotic fluid surrounding the fetus. Both CVS and amniocentesis analyze the baby’s chromosomes directly, providing a definitive diagnosis. While ultrasound findings are indicators, these genetic tests provide the confirmation needed.