Main findings
To the best of our knowledge, this is the first study to estimate the association between ApoB, ApoB/ApoA-1 ratio during mid-pregnancy and the risk of PTB. We found that maternal mid-pregnancy ApoB level and ApoB/ApoA-1 ratio were positively related to the risk of PTB. In addition, these associations showed a linear dose-response relationship. Subgroup analysis suggested that the effect of ApoB/ApoA-1 ratio might be more relevant among women with pre-pregnancy BMI ≤ 24 kg/m2, age at delivery ≥ 35 years or those who were multiparous.