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.