Trisomy 18, also known as Edwards Syndrome, is a serious genetic condition that affects many parts of a developing baby’s body. It is a chromosomal disorder associated with severe physical abnormalities and intellectual disability. Families often have questions about how this condition affects development, including the baby’s activity level while still in the womb. Understanding the biological underpinnings of this syndrome provides clarity regarding the typical course of these pregnancies.
The Genetic Basis of Trisomy 18
Trisomy 18 is caused by the presence of an extra copy of the eighteenth chromosome in a baby’s cells, meaning there are three copies instead of the usual two. This extra genetic material disrupts the normal blueprint for development, leading to the condition’s characteristic features. The error that causes this extra chromosome is most often a random event called non-disjunction.
Non-disjunction occurs when chromosome pairs fail to separate properly during the formation of the egg or sperm cell. If a reproductive cell with three copies of chromosome 18 is involved in fertilization, the resulting embryo will have the full form of the condition, known as complete Trisomy 18, which accounts for the vast majority of cases. Less common variants include mosaic Trisomy 18, where only some cells have the extra chromosome, and partial Trisomy 18, where only a segment of the chromosome is duplicated. The resulting genetic imbalance drives the severe developmental issues and subsequent growth limitations observed during pregnancy.
Fetal Movement and Growth Restrictions
Babies with Trisomy 18 generally show significantly reduced fetal movement compared to typically developing babies. This diminished activity is often an early indicator of underlying health challenges, stemming from neurological and muscular involvement associated with the syndrome. The movements that do occur are often less vigorous and less frequent, which can be noted during prenatal monitoring.
The extra chromosome 18 also commonly results in intrauterine growth restriction (IUGR), meaning the fetus measures substantially smaller than expected for the gestational age. This growth restriction is often evident early in the pregnancy and becomes more pronounced in the second and third trimesters. The combination of severe structural anomalies and poor growth contributes to the reduced physical activity observed in the womb.
Amniotic fluid volume abnormalities are also frequently associated with Trisomy 18 pregnancies. Polyhydramnios (excess amniotic fluid) is a common finding, sometimes linked to the baby’s difficulty swallowing the fluid. Conversely, oligohydramnios (low amniotic fluid) may occur due to associated renal or urinary tract problems. These fluid imbalances further complicate the intrauterine environment.
Key Structural Findings During Pregnancy
Prenatal ultrasound examinations often reveal specific physical anomalies that strongly suggest Trisomy 18. Nearly all fetuses with the condition have severe, life-limiting congenital heart defects. Common cardiac findings include ventricular septal defects (VSD), atrial septal defects (ASD), and patent ductus arteriosus (PDA).
The syndrome is also characterized by distinct abnormalities in the limbs and skeleton. A classic finding is a clenched hand posture, where the fingers are permanently flexed and often overlap. The feet may also present as “rocker-bottom feet,” characterized by a prominent heel and a rounded sole. These skeletal malformations contribute to the reduced range of motion and limited activity level of the baby.
Other internal organ systems are commonly affected. Kidneys may show abnormalities like a horseshoe kidney or other renal malformations. Brain abnormalities can also be present, and choroid plexus cysts are frequently noted on ultrasound. Additionally, defects like omphalocele (where abdominal organs protrude into the umbilical cord) and a single umbilical artery are associated with the condition.
Confirmation and Postnatal Outlook
If screening tests suggest the possibility of Trisomy 18, a definitive diagnosis requires cytogenetic analysis. Screening tests, such as Non-Invasive Prenatal Testing (NIPT) or maternal serum markers, assess the probability of the condition. Diagnostic procedures, like amniocentesis or chorionic villus sampling (CVS), analyze the baby’s cells to confirm the presence of the extra chromosome 18.
Trisomy 18 is a life-limiting diagnosis, and the prognosis is challenging. A large percentage of pregnancies diagnosed prenatally do not survive to term, often resulting in miscarriage or stillbirth. For those babies born alive, the median life expectancy is very short, often measured in days or weeks, due to severe organ malformations, particularly heart and respiratory issues.
A small fraction of babies (approximately 5% to 10%) may survive beyond their first year, with girls having a statistically higher chance of survival than boys. Management after birth often involves immediate, specialized medical support. Many families choose palliative or comfort care to ensure the baby’s quality of life, rather than aggressive measures to prolong survival.