Strengths and Limitation
One of the major strengths of this study is the sample size. For this cohort, we recruited 669101 participants and followed up pregnancy outcomes with strict quality controls. The number of each category of pre-pregnancy BMI and pregnancy outcomes were enough that multivariable regression models were not over-fitted10. In fact, this is the first study from China using Chinese BMI classification to look at each separate category of pre-pregnancy BMI, therefore enabling the observation of a much clearer association of pre-pregnancy BMI with risk for several outcomes. Additionally, it is first time to examine the association of pre-pregnancy BMI with several outcomes according to maternal age group, making the results more practical for risk assessment and counselling before pregnancy.
The study has some limitations. First, although all of outcomes are abstracted from the medical records that is high credibility and accuracy, the outcomes are limited to an gestational age of 28 weeks and over, which may exist selection bias and underestimate the prevalence of several outcomes (PTB, caesarean delivery, shoulder dystocia or birth injury and stillbirth), thus may underestimate the association of pre-pregnancy BMI with the outcomes listed.32 Second, some important information on pregnancy complications and obstetrics were missing too much or not collected in the NFPHEP. For example, data on gestational hypertension and diabetes were missing in 99.1% of participants due to low rate of screening, and data on gestational weight gain and causes of PTB (spontaneous versus indicated) were not collected, all of which make the interpretation of our results difficult. Thus, further studies are warranted to fully understand the association of pre-pregnancy BMI with adverse pregnancy outcomes. Furthermore, we were not able to examine the mediated effects of family income on the association of pre-pregnancy BMI with adverse pregnancy outcomes as such data also was missing for the vast majority of the participants. However, the adjusted RRs did not change substantially after additionally adjusting for economic pressure, which is correlated with family income.33 Third, we may have underestimated the associations of BMI with adverse pregnancy outcomes because some policy interventions implemented in China, such as maternal system health care policy that has covered more than 95% pregnant and monitored several risk factors for adverse outcomes during the pregnancy.32 Additionally, although we examined the associations of pre-pregnancy BMI with several adverse outcomes according to maternal age, the number of participants who had adverse pregnancy outcomes in 40-50 years group was not enough to calculate the precise adjusted RRs with precise confidence interval. Finally, the socio-demographic characteristics, economic, culture, nutritional models and medical service level might not be representative of other countries and regions, suggesting that results from the present study should be validated in different population.