Pre-natal brain development of fetuses with a myelomeningocele.

Biometric measurements of 21 fetuses referred to the Fetal Management Unit at St. Mary's in Manchester with a diagnosis of myelomeningocele were analysed. Five fetuses had more than 3 sets of measurements carried out, the last of which were performed during the 3rd trimester. The majority of the head circumference measurements were on or below the 3rd percentile for normal head size whilst almost all of the cerebral ventricular atrial measurements were more than 10 mm, a figure generally accepted as being at the upper limit of normal, measurements above this indicate that ventriculomegaly is present. In normal fetuses the atrial measurement is constant at or below 10 mm throughout gestation, but in the fetuses with a myelomeningocele the atrial measurements got progressively larger as gestation proceeded. Because their head circumferences did not increase disproportionately quickly, there was no evidence that the fetuses were developing hydrocephalus in utero, rather the large atrial measurements were indicative of poor development of the cerebral hemispheres. These findings together with the Arnold-Chiari malformation and the abnormalities in the spinal cord indicate that the central nervous system in its entirety is abnormally developed in fetuses with a myelomeningocele.