Small fetal size: a risk factor for breech birth at term.

OBJECTIVE: To examine fetal size as a risk factor for breech birth at term.

METHODS: Singleton breech or cephalic births of gestational age > or = 37 weeks in New South Wales (NSW), Australia from 1990 to 1996 were analyzed. Birthweight percentile was used as a measure of fetal size at the time of birth. Factors associated with breech birth at term were analyzed using logistic regression.

RESULTS: There were 18914 singleton breech and 540164 cephalic births in the study period. The important independent predictors of breech birth at term were advancing maternal age, primiparity, female sex and small size for gestational age. Infants < 10th percentile had an adjusted odds ratio of 1.33 (95% CI 1.28-1.38) for breech birth at term compared with 25th-75th percentile infants.

CONCLUSIONS: Breech birth at term was associated with smaller fetal size for gestational age. This was shown directly through an association with birthweight-for-gestational-age percentiles and indirectly through association with female sex, primiparous birth and congenital anomalies.