Interpreting ultrasound measurements in preterm pregnancies complicated by premature rupture of the membranes is problematic; the effects of physical compression and diminished fetal growth are unclear. To delineate these factors, we analyzed ultrasound scans from 397 preterm pregnancies, including 26 with premature rupture of the membranes. Mean biparietal diameter (BPD), abdominal circumference, femur length, and estimated fetal weight were all significantly smaller with premature rupture of the membranes than without. However, this reflected younger gestational age and diminished fetal growth, rather than physical compression. The reduction in abdominal circumference was greater than that in other indices, which further suggests compromised intrauterine growth. Clinical interpretation of ultrasound measurements should consider the increased likelihood of growth retardation with preterm premature rupture of the membranes to avoid underestimating gestational age and/or missing the diagnosis of intrauterine growth retardation.