Editor—We report a case in which a termination of pregnancy for fetal abnormality at 18 weeks' gestation showed a supernumerary marker chromosome. This extra chromosome did not hybridise to any alphoid probes and was found to have a chromosome 3 origin when investigated by M-FISH.

An anomaly ultrasound scan was performed because of raised alphafetoprotein and beta HCG levels at 17 weeks' gestation in a 32 year old, primigravida mother. The scan showed a large and cystic left kidney, banana sign, and absent cisterna magna, and signs of an open sacral spina bifida. The pregnancy was terminated and necropsy showed a male fetus consistent with 18 weeks' gestation with no dysmorphic facial features. A high arched palate with a small amount of postnuchal oedema was noted as well as a single transverse palmar crease on the right hand. Inspection of the back showed a 1.3 cm long lumbosacral myelomeningocele with protruding lower lumbar spinal cord. On internal examination the cerebral hemispheres were fully cleaved and appeared fluctuant suggesting the possibility of internal hydrocephalus. The posterior fossa of the brain was reduced in anteroposterior diameter as well as appearing deep and funnel shaped, and the extension of the cerebellar tonsils was below the level of the foramen magnum. These findings are consistent with Arnold-Chiari malformation. There was marked asymmetry of the kidneys; the right kidney showed normal fetal lobation and shape but the left kidney was very large and had thin, translucent, subcapsular cysts, especially at the lower pole. The cut surface showed a poor demarcation between the cortex and medulla and the presence of cysts in most of the renal parenchyma. These findings are consistent with cystic renal dysplasia. The placenta was unremarkable and the cord had three normal blood vessels.

The chromosomes of the abortus were examined …