Objective: To test whether being small for gestational age, defined as having a birthweight less than the 10th centile of intrauterine growth references, is a risk factor for preterm delivery for singleton live births.
Design: A case-control study.
Setting: Maternity hospitals in 16 European countries.
Sample: Four thousand and seven hundred preterm infants between 22 and 36 completed weeks of gestation and 6,460 control infants between 37 and 40 weeks of gestation.
Methods: Newborn babies are identified as being small for gestational age using customized reference standards derived from models of fetal growth. The impact of being small for gestational age on preterm delivery is estimated using logistic regression.
Main outcome measure: Spontaneous or induced preterm delivery.
Results: Being small for gestational age is significantly associated with preterm birth, although the magnitude of this association differs greatly by type of delivery and gestational age. Over 40% of induced preterm births for reasons other than the premature rupture of membranes are small for gestational age compared with 10.7% of control infants (OR 6.41). For spontaneous or premature rupture of membranes related preterm births, the association is also significant, but weaker (OR 1.51). The relationship between growth restriction and preterm delivery is strongest for preterm births before 34 weeks of gestation.
Conclusions: These findings highlight the phenomenon of abnormal fetal growth in all premature infants and, in particular, infants delivered by medical decision for reasons other than premature rupture of membranes. The observed association between being small for gestational age and preterm delivery among spontaneous preterm births merits further attention because the causal mechanisms are not well understood.