Breech babies were shown to have a smaller mean biparietal diameter (BPD) neonatally compared with that of a matched group of vertex babies. This was due to a mild skull deformation which occurred in at least one-third of 100 consecutive term breech babies examined. Features of this skull deformation (dolichocephaly, a prominent occiput with a suboccipital shelf, an elongated face and a parallel-sided head) constitute the 'breech head'. The caliper-determined occipitofrontal/biparietal diameter ratio (OFD/BPD) in these newborn infants was consistently above 1 . 30. This ratio, when calculated from sonar examination in the third trimester, was also found to correlate well with the 'breech head' shape. The identification by ultrasound of these babies should prevent the misdiagnosis of fetal growth retardation based on serial BPD measurements alone.