Comparison of crown-rump length and biparietal diameter measurements with prior work. Crown-rump length (CRL) and biparietal diameter (BPD) are shown as a function of gestational age. Open symbols show the results obtained for each conceptus in the current study. The solid lines were generated using the regression equations shown in Table 1. Solid symbols show results from prior work. Values were calculated using formulas for CD-1 embryos obtained using 7.5–10 MHz (blue squares; ) or 15 MHz (green triangles; ) ultrasound or are means reported in Tables for C57Bl/6 embryos (red circles , purple diamonds ).