An observational study in Pregnancy, Obesity and Gestational Hypertension, sponsored by Shanghai First Maternal and Infant Health Hospital affiliated to Tongji University Medical School. Completed at 1 site in China. Open to female participants aged 18 Years to 45 Years. Per ClinicalTrials.gov, last updated 2026-11-06.
Sponsored by Shanghai First Maternal and Infant Health Hospital affiliated to Tongji University Medical School · Observational
This study asks whether the current weight-gain guideline for pregnant people with obesity should be lowered, and whether the answer depends on how severe the obesity is. The guideline from the Institute of Medicine (IOM) recommends that all pregnant people with obesity gain 5.0 to 9.0 kg, regardless of obesity class. However, growing evidence suggests that people with Class II or III obesity may benefit from gaining less than this range-or even losing a small amount of weight-without harming the baby. A randomized trial to test this directly would be ethically and practically difficult, so this study uses a different approach: it reconstructs what such a trial would have shown, using nationwide U.S. birth records.
The study uses 2024 U.S. birth certificate data for people with pre-pregnancy obesity who had a single live birth. It compares three weight-gain patterns during pregnancy: (1) net weight loss (less than 0 kg gained), (2) restrictive gain (weight gained, but below the dynamic, gestational-age-specific IOM lower bound), and (3) standard gain within the IOM-recommended range. The main questions are whether restrictive gain or weight loss, compared with standard gain, reduces the risk of hypertensive disorders of pregnancy (such as gestational hypertension and preeclampsia) while not increasing the risk of a baby born small for gestational age (SGA). The study also examines whether the balance of benefits and harms differs across Class I, Class II, and Class III obesity.
This is an observational study, not a randomized trial. It does not enroll participants or provide treatment. All analyses use existing, de-identified public health data. Results are intended to inform whether future weight-gain guidelines should provide class-specific recommendations rather than a single range for all obesity classes.
Detailed Description
Background. The 2009 IOM guidelines recommend a uniform gestational weight gain (GWG) target of 5.0-9.0 kg for all pregnant people with obesity, without distinguishing between Class I (BMI 30.0-34.9 kg/m²), Class II (BMI 35.0-39.9 kg/m²), and Class III (BMI ≥ 40.0 kg/m²) obesity. This single-range approach is increasingly questioned because the cardiometabolic consequences of obesity are not uniform across severity strata. For individuals with Class II-III obesity, restrictive gain or modest gestational weight loss may reduce maternal hypertensive disorders of pregnancy (HDP) and normalize fetal overgrowth (large for gestational age, LGA), while maternal adipose reserves may protect against small-for-gestational-age (SGA) birth. A pragmatic randomized trial assigning pregnant people to gestational weight loss raises ethical and clinical equipoise concerns, and conventional observational analyses of GWG are vulnerable to gestational-age-related survival bias and baseline confounding.
Objectives. This study applies a formal target trial emulation framework to estimate the causal effect of three GWG strategies on a co-primary benefit-risk outcome pair: maternal HDP (efficacy) and fetal SGA (safety). A key secondary outcome is LGA. The central hypothesis is that the comparative benefit-risk profile of restrictive gain or weight loss versus standard IOM gain differs meaningfully across obesity classes, particularly in Class III obesity.
Design and data source. This is a retrospective, nationwide target trial emulation using the U.S. National Vital Statistics System (NVSS) Natality Public Use Files. The primary emulation cohort is drawn from the 2024 singleton live-birth cohort. A 10-year physiological reference cohort (2015-2024), restricted to uncomplicated pregnancies, is used to standardize GWG into race- and gestational-age-specific Z-scores.
Emulated strategies. To eliminate gestational-age-dependent survival bias, dynamic, gestational-age-specific IOM bounds are calculated for each delivery week from 24 to 41 weeks: a first-trimester gain of 1.1 kg plus a second/third-trimester velocity of 0.17 kg/week (lower bound) to 0.27 kg/week (upper bound). Participants are assigned to one of three strategies: Gestational Weight Loss (net GWG \< 0.0 kg); Restrictive Gain (GWG ≥ 0.0 kg but below the dynamic IOM lower bound); or Standard IOM Gain (GWG within the dynamic IOM range; reference group). Individuals exceeding the dynamic IOM upper bound are not included in the three-arm comparison, because the study is designed to ask whether guidelines should be lowered, not whether they should be exceeded.
Causal estimation. Within each obesity class, generalized propensity scores for the three strategies are estimated via multinomial logistic regression, conditioned on continuous pre-pregnancy BMI and prespecified sociodemographic and clinical confounders. Propensity score overlap weighting (targeting the average treatment effect in the overlap population, ATO) is used to achieve covariate balance while avoiding extreme weight variance. ATO-weighted modified Poisson regression with robust sandwich variance estimators is used to estimate relative risks and absolute risk differences for HDP, SGA, and LGA. Risk differences are translated into numbers needed to treat for HDP/LGA prevention and numbers needed to harm for SGA induction.
Secondary and sensitivity analyses. Prespecified effect modification is evaluated within the Class III stratum across parity, race/ethnicity, and maternal age, with ATO weights re-estimated de novo in each subgroup. A biological negative control outcome (infant male sex) is analyzed to detect residual structural confounding. Sensitivity analyses include alternative first-trimester gain assumptions (0.0 kg and 2.0 kg), E-value quantitative bias analysis for statistically significant causal estimates, and multiple imputation by chained equations to address missing data, alongside the primary complete-case analysis.
Reproducibility. Data extraction, physiological Z-score modeling, and cohort construction are performed in Python 3.9.6. Overlap weighting, modified Poisson regression, marginal effect extraction, and E-value calculation are performed in R (v4.5.1).
11,522 studies on the registry are indexed under Obesity; 2,033 are open to participants now.
This study's enrollment of 205,636 is above the median of 114 across 2,154 observational studies indexed under Obesity.
Browse Obesity studies →Shanghai First Maternal and Infant Health Hospital affiliated to Tongji University Medical School is the lead sponsor of 3 studies on the registry; 1 is open to participants now.
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The study population comprises pregnant individuals with pre-pregnancy obesity (BMI ≥ 30.0 kg/m²) who had a singleton live birth in the United States in 2024, as recorded in the NVSS Natality Public Use File. Eligible participants initiated prenatal care in the first trimester, delivered between 24 and 41 completed weeks of gestation, and were aged 18-45 years at delivery. The cohort is stratified into three pre-pregnancy obesity classes: Class I (BMI 30.0-34.9 kg/m²), Class II (BMI 35.0-39.9 kg/m²), and Class III (BMI ≥ 40.0 kg/m²). Individuals with pre-existing diabetes or chronic hypertension, missing core outcome data, major congenital anomalies, or implausible GWG values are excluded. A separate 9-year (2016-2024) uncomplicated pregnancy reference cohort from the same NVSS database is used exclusively to derive race- and gestational-age-specific physiological GWG Z-scores; it is not part of the primary target trial emulation cohort.
Go through the criteria one by one — yes, no or not sure — and take the list to your doctor or the study team. Kept in this browser only.
Exclusion Criteria:
Underlined terms in the criteria open these too. Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Weight in kilograms divided by height in metres squared. Example: 70 kg and 1.75 m gives 70 ÷ (1.75 × 1.75) ≈ 22.9.
What counts as overweight or obesity differs between countries (some use lower cut-offs), so go by the range the study states.
BMI (body mass index)
Weight in kilograms divided by height in metres squared. Example: 70 kg and 1.75 m gives 70 ÷ (1.75 × 1.75) ≈ 22.9.
What counts as overweight or obesity differs between countries (some use lower cut-offs), so go by the range the study states.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Participants with net gestational weight gain (GWG) \< 0.0 kg during pregnancy.
Other: Gestational Weight Gain (GWG) Strategy
Participants with net GWG ≥ 0.0 kg but strictly below the dynamic, gestational-age-specific IOM lower bound.
Other: Gestational Weight Gain (GWG) Strategy
Participants with net GWG within the dynamic, gestational-age-specific IOM recommended range \[Lower Bound, Upper Bound\]. This is the reference group.
Other: Gestational Weight Gain (GWG) Strategy
The study exposure is the pattern of total gestational weight gain (GWG) during pregnancy, categorized into three mutually exclusive strategies based on dynamic, gestational-age-specific IOM thresholds: Gestational Weight Loss (GWG \< 0 kg), Restrictive Weight Gain (GWG ≥ 0 kg but below the IOM lower bound), and Standard IOM Gain (GWG within the IOM recommended range).
Hypertensive Disorders of Pregnancy (HDP)
A composite binary endpoint of incident gestational hypertension or preeclampsia/eclampsia during pregnancy.
Time frame: Up to delivery
Small for Gestational Age (SGA)
Neonatal birthweight \<10th percentile for gestational age, stratified by infant sex and maternal parity using validated U.S. national birthweight reference tables.
Time frame: At birth.
Large for Gestational Age (LGA)
Neonatal birthweight \>90th percentile for gestational age, sex, and parity, evaluating the normalization of maternal obesity-driven fetal overgrowth.
Time frame: At birth.
Infant Male Sex
Evaluated as a biological negative control outcome to detect potential structural or unmeasured baseline confounding.
Time frame: At birth.
Plan to share: No — Individual participant data (IPD) will not be shared by the investigators because the source data are de-identified public-use files from the U.S. National Center for Health Statistics (NCHS), National Vital Statistics System (NVSS) Natality Public Use Files. These data are publicly available to all researchers through standard NCHS access procedures, and the investigators do not have authority to redistribute NVSS records. To promote transparency and reproducibility, the analytic code (including Python and R scripts for cohort construction, physiological GWG Z-score standardization, propensity score overlap weighting, modified Poisson regression, and sensitivity analyses), variable definitions, and data dictionary will be made available from the corresponding author upon reasonable request and will be deposited in a public repository upon publication. These materials will be shared with researchers for the purpose of replication, methodological verification, or secondary methodologica
From the registry record's own update history. This site started tracking changes on Sep 25, 2026; for anything earlier, see the record history on ClinicalTrials.gov ↗
This study is completed, as verified in Oct 2026. You cannot join it, but the record below documents what was studied.
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Shanghai First Maternal and Infant Health Hospital affiliated to Tongji University Medical School