Objectives: The aim of this study was to compare ultrasound fetal size at high altitude and sea level.
Methods: Three hundred and thirty-four women in Cerro de Pasco at 4300 m (14,100 ft) altitude and 278 women in Lima (sea level) were recruited to the study. Ultrasound fetal biometry was carried out between 14 and 42 weeks of gestation. Biparietal diameter, occipitofrontal diameter, abdominal circumference and femur length were measured and head circumference and estimated fetal weight were derived from these data. Two hundred and seventy-seven women (82.9%) in Cerro de Pasco and 216 (77.7%) in Lima had normal singleton pregnancies and certain menstrual dates. These women were selected for statistical analysis. Fractional polynomial regression analysis on gestational age was performed, controlling for maternal height and parity.
Results: Fetal biometry measurements were significantly smaller in Cerro de Pasco compared with Lima. When gestation bands were compared this effect was present from 25 to 29 weeks onwards, and was greater in the abdominal circumference than in the head circumference and femur length (ratios Cerro de Pasco: Lima, 0.96, 0.97 and 0.98, respectively). Estimated fetal weight was also significantly lower in Cerro de Pasco (ratio 0.88), as were birthweights (ratio 0.88). If the centiles derived from the Lima population were applied for Cerro de Pasco, 11.2% of all estimated fetal weights would be below the fifth centile, and 1.08% above the ninety-fifth.
Conclusions: These data suggest that at high altitude, all fetal biometry measurements follow a lower trajectory than at sea level. Specific biometry charts should therefore be used for obstetric ultrasound at high altitude.