The fetal cisterna magna

Antenatal sonography routinely images the fetal cisterna magna. The identification of a prominent cisterna magna on a routine antenatal sonogram performed for obstetric indications may raise the question of a congenital posterior fossa lesion. To delineate the size of the fetal cisterna magna, standardized measurements for the midsagittal depth of the cisterna magna were prospectively obtained on 219 consecutive antenatal sonograms in fetuses of 15 menstrual weeks or more. The cisterna magna depth measured 5 +/- 3 mm; the largest cisterna magna measured 10 mm in depth. In the absence of other findings to suggest a posterior fossa lesion, a prominent cisterna magna is unlikely to be clinically significant.     

Minimal fetal renal pyelectasis

To assess the possible relationship between the degree of maternal hydration and the sonographic identification of minimal fetal renal pyelectasis, a prospective study was performed in which fetuses demonstrating mild dilation of the renal pelvis (maximum diameter ranging from 3 to 11 mm) were reexamined after the mothers refrained from oral intake for 12 hours. Complete or almost complete resolution of the pyelectasis occurred in only four of 17 kidneys (23.5 per cent) while the remaining fetal kidneys demonstrated little or no change in the degree of pyelectasis following maternal dehydration. This observation, as well as previous experimental research, suggests that the state of maternal oral hydration is not a major cause of minimal fetal pyelectasis. Additionally, the observation of fetal pyelectasis measuring at least 3 mm in greatest dimension is common, occurring in approximately 18 per cent of fetuses older than 24 menstrual weeks.