Ontogeny of clenched-hand development in trisomy 18 fetuses: A serial transabdominal fetoscopic observation.

Malpositioning of the fingers is a characteristic anomaly in fetuses with trisomy 18. The defect results in part from muscle variations along the radial margin of forearm and hand, absence of the thenar muscles, anomalous tendons and attachments among the forearm groups, and fusions among the arm flexor group. These variations result in radial or ulnar displacement of the tendons of extensor digitorum and digiti minimi, with overlapping of the fourth and fifth fingers radially and second finger in an ulnar direction. The ontogeny of these changes is unknown. We performed serial transabdominal thin-gauge fetoscopy in a patient with increased nuchal thickening at 12 weeks of pregnancy at the time of genetic testing and again at 14 weeks at the time of termination of pregnancy. Changes in the positioning of the fingers were not apparent at 12 weeks, but were evident at 14 weeks. The findings were beyond the resolution of ultrasound. We conclude that malpositioning of the fingers in trisomy 18 occurs some time between 12 and 14 weeks of gestation. Noninvasive confirmation of these findings may be possible with new and improved ultrasound imaging capabilities or perhaps with three-dimensional ultrasound.