The authors used ultrasound examination in an attempt to identify fetal and environmental differences between breech and vertex presentations. During a period of 13 months, 228 pregnancies with the fetus in breech position were found by routine ultrasound screening in the 33rd gestational week. Before delivery, 132 fetuses (58%) turned to vertex presentation and 96 (42%) remained in breech presentation. Fetal growth, posture, position, placental site, and amniotic fluid volume in the 33rd, 35th, and 38th gestational weeks were studied. In contrast to earlier proposed theories, no difference between the two groups (breech and vertex) was found regarding the frequency of extended fetal legs and cornual-fundal placental implantation. However, the neonates born in breech presentation had a shorter gestational age by ten days at delivery, and preterm delivery was more common. At birth, the breech neonates weighed 4.9% less than their vertex controls in relation to gestational age. In the breech group, there was an increased frequency of oligohydramnios, contracted pelvis, and uterine and fetal malformations. These conditions occurred only in 15% of the breech pregnancies; in the remaining 85%, no single cause of the breech presentation could be identified.