Weight-gain-for-gestational-age z-score in twin pregnancies in China and its association with preterm birth: a retrospective cohort study.

To evaluate the association between the weight-gain-for-gestational-age z-score and preterm birth (PTB) in a large Chinese twin pregnancy cohort. A hospital-based retrospective study included 2832 twin pregnancies between January 1, 2015, and December 31, 2019. The primary composite outcome was defined as PTB. The total gestational weight gain was converted into z-score, and logistic regression models were used to examine the associations between z-scores and PTB. We developed a weight-gain-for-gestational-age z-score to determine the gestational weight gain during twin pregnancies in Chinese women. Low (<P25) and high (≥P75) z-scores were associated with a higher risk for spontaneous PTB (SPTB) than scores between the 25th and 75th percentiles (adjusted odds ratio [OR]: 1.35, 95% confidence interval [CI]: 1.05-1.74; 1.28, 0.99-1.66; respectively). Similarly, low z-scores (1.40, 1.12-1.76) and high z-scores (1.97, 1.57-2.47) were also associated with an increased risk for iatrogenic PTB (IPTB). Normal weight subgroups exhibited comparable outcomes. In addition, z-scores between the 25th and 75th percentiles indicated an equivalent total gestational weight gain at 37 weeks of 15.8-20.9 kg for underweight, 14.6-20.3 kg for normal weight, and 11.8-18.6 kg for overweight or obese women, which are narrower than the 2009 IOM recommendation. Our findings indicate that inadequate gestational weight gain, as evaluated by the z-score, is associated with a significantly increased risk for SPTB, whereas inadequate and excessive gestational weight gain are associated with a significantly increased risk for IPTB, in a large Chinese twin pregnancy cohort.