Ultrasound measurement of biparietal diameter and umbilical artery blood flow in the normal fetal guinea pig.

Background and Purpose: Measurement of the biparietal diameter (BPD) by use of B-mode ultrasound provides a useful means for assessment of gestational age and brain growth in humans during pregnancy. Recording of flow velocity waveforms from the umbilical artery, using Doppler ultrasound, is used to assess development of the fetal placental circulation. We sought to measure these ultrasound parameters during normal pregnancy in the guinea pig and develop normative data.
Methods: Measurements of BPD were made on 205 fetuses of various gestational ages; 114 fetuses had 2 or more serial studies performed (total n = 474).
Results: BPD increased from 0.806cm at 22 to 26 days, to 1.922cm at term (69 days), (y = -0.00043x2 + 0.06881x - 0.75941, with an r value of 0.995, where x = days' gestation, y = biparietal diameter [cm]). Umbilical artery flow velocity waveform resistance index (RI) decreased as gestation advanced (y = -0.012x + 1.294 with an r value of 0.887, where x = days gestation, y = RI) reflecting expansion of the placental vascular bed.
Conclusions: It is possible to use ultrasound to study pregnancy in the guinea pig. The BPD may be used to estimate gestational age. Resistance to blood flow in the placenta may be assessed using the RI derived from the umbilical artery flow velocity waveform.