PTB= preterm birth (< 37 weeks’ gestational age, GA); Late PTB (32-36 weeks’ GA); Very PTB (<32 weeks’ GA); extreme PTB (<28 weeks’ GA); ^a = adjusted analysis included the following variables: maternal country of birth, maternal age at index birth, parity, maternal educational degree, maternal employment, pregnancy conceived with assisted reproductive technology, sex of the child. Observation with missing variables were excluded from the model. The Lazio Region is not included in adjusted analyses.