Outcome measures
According to the World Health Organization, PTB was defined as gestational length ≤ 37 completed weeks (24). Moderate PTB (MPTB) was defined as gestational length between 32 and 36 completed weeks. Very PTB (VPTB) was defined as gestational length < 32 completed weeks. The VPTB included extremely PTB (gestational length < 28 completed weeks).