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CAR: Anomalies - Chromosome
Patau's Syndrome (Trisomy 13)

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Introduction Antenatal Postnatal West Midlands Data


Trisomy 13 is the most severe viable trisomy caused by an additional copy of chromosome 13. Most pregnancies will result in miscarriage but some survive the first few weeks of life. As with other trisomies such as Down's syndrome there is a maternal age effect with an increased incidence in older mothers.

Additional structural anomalies are common, particularly facial anomalies (midline clefts, hypotelorism, microphthalmia, and anophthalmia) arising from structural anomalies of the brain, frequently microcephaly and holoprosencephaly. Other associated anomalies include cardiac, renal, and intestinal (diaphragmatic hernia) anomalies. Characteristic features include low set ears, post-axial polydactyly, flexion contractures, rocker bottom feet, scalp defects, and haemangiomas.

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As with trisomy 18 many fetuses affected by trisomy 13 will show early intrauterine growth failure, and will measure smaller than expected at the first scan. Major structural anomalies are occasionally identified in the late first or early second trimester, but the most common initial presentation is for the small measurement to be interpreted as incorrect dating of the pregnancy. Subsequent scans again indicating poor fetal growth indicate a more serious underlying problem.
Other associated anomalies will be visible on ultrasound at 20 weeks including facial clefts, holoprosencephaly, cardiac defects, diaphragmatic hernia, and hexadactyly. The discovery of a structural anomaly in association with growth failure increases the chances of chromosomal anomalies, and women should be offered amniocentesis or other invasive testing to establish the fetal karyotype. Following a confirmed prenatal diagnosis, the decision to be made is either to terminate the pregnancy, or to manage it conservatively.

If a fetus with significant structural anomalies dies in-utero, the medical staff should be suspicious of a chromosomal anomaly. In women who have elected not to have invasive testing the question should be asked again, as there is a much higher success rate with amniocentesis, than waiting for post delivery cultures, which often fail due to bacterial over-growth. In contrast, fetal cells can usually be cultured, even several days after intra-uterine death.

Couples who have suffered the loss of a child with Patau's syndrome should be offered invasive testing in any future pregnancy. The recurrence risk is small, and is not precisely known and many couples will decline in the knowledge that ultrasound can give a degree of reassurance for this condition, and that invasive testing carries a small risk of pregnancy loss.

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There is a high rate of attrition of affected pregnancies and many abort spontaneously. Trisomy 13 is reported to be as common in spontaneous abortions as trisomy 18. Most affected live born infants die within the first weeks of life. Some cases with mosaic trisomies have survived for several years, all affected by severe mental retardation.

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© Perinatal Institute 2011