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.