Association between maternal pre-pregnancy obesity and preterm birth according to maternal age and race or ethnicity: a population-based study

Abstract

Background: The relation between maternal pre-pregnancy obesity and preterm birth is controversial and inconclusive. We aimed to clarify the association between pre-pregnancy obesity and preterm birth by maternal age and race or ethnicity in a large, multiracial, multiethnic, and diverse population in the USA.

Methods: We did a population-based cohort study using nationwide birth certificate data from the US National Vital Statistics System for 2016 and 2017. We included all mothers who had a live singleton birth and who did not have pre-existing hypertension or diabetes. Pre-pregnancy obesity was defined as a pre-pregnancy BMI of at least 30 kg/m². Preterm birth was defined as gestational age of less than 37 weeks. We used logistic regression models adjusted for maternal age, race or ethnicity, parity, education levels, smoking during pregnancy, previous history of preterm birth, marital status, infant sex, and timing of initiation of prenatal care to estimate the odds ratio (OR) of preterm birth.

Findings: We included 7 141 630 singleton livebirths in our analysis, 527 637 (7·4%) of which were preterm births. 127 611 (7·5%) Hispanic mothers, 244 578 (6·6%) non-Hispanic white mothers, and 102 509 (10·4%) non-Hispanic black mothers had preterm births. In the overall population, maternal pre-pregnancy obesity was significantly associated with an increased risk of preterm birth compared with maternal pre-pregnancy healthy weight (ie, BMI of 18·5-24·9 kg/m²; adjusted OR 1·18 [95% CI 1·18-1·19]). In non-Hispanic white women, maternal obesity was inversely associated with preterm birth among those younger than 20 years (adjusted OR 0·92 [95% CI 0·88-0·97]), but positively associated with preterm birth among those aged 20 years or older (1·04 [1·01-1·06], 1·20 [1·18-1·23], 1·34 [1·31-1·37], 1·40 [1·36-1·43], and 1·39 [1·31-1·46] among those aged 20-24 years, 25-29 years, 30-34 years, 35-39 years, and ≥40 years, respectively). In Hispanic women, maternal obesity was not associated with preterm birth among those younger than 20 years (0·98 [0·93-1·04]), but positively associated with preterm birth among those aged 20 years or older (1·06 [1·03-1·09], 1·21 [1·17-1·24], 1·32 [1·28-1·36], 1·38 [1·33-1·43], and 1·30 [1·22-1·40] among those aged 20-24 years, 25-29 years, 30-34 years, 35-39 years, and ≥40 years, respectively). In non-Hispanic black women, maternal obesity was inversely associated with preterm birth among those younger than 30 years (0·76 [0·71-0·81] in those <20 years, 0·83 [0·80-0·86] in those aged 20-24 years, and 0·98 [0·95-1·01] among those aged 25-29 years), but positively associated with preterm birth among those aged 30 years or older (1·15 [1·11-1·19], 1·26 [1·20-1·32], and 1·29 [1·18-1·42] among those aged 30-34 years, 35-39 years, and ≥40 years, respectively).

Interpretation: Maternal pre-pregnancy obesity is significantly associated with the risk of preterm birth in the general population, but the risk differs according to maternal age and race or ethnicity. Future investigation is warranted to understand the underlying mechanisms.

Funding: US National Institutes of Health.

Figure 1:. Participant flow chart

NVSS=US National Vital Statistics System.

Figure 2:. Association between pre-pregnancy obesity and risk of preterm birth in Hispanic (A), non-Hispanic white (B), non-Hispanic black (C), and other racial or ethnic (D) populations, by maternal age group

Data are from the US National Vital Statistics System for 2016 and 2017. Parity, education level, smoking during pregnancy, previous history of preterm birth, marital status, infant sex, and timing of initiation of prenatal care were adjusted for in models. Women who had a healthy pre-pregnancy BMI (ie, 18·5–24·9 kg/m²) were the reference group. Error bars represent 95% CIs. OR=odds ratio.

Figure 3:. Joint association of maternal age and pre-pregnancy obesity status with risk of preterm birth in Hispanic (A), non-Hispanic white (B), non-Hispanic black (C), and other racial or ethnic (D) populations

Data are from the US National Vital Statistics System for 2016 and 2017. Parity, education level, smoking during pregnancy, previous history of preterm birth, marital status, infant sex, and timing of initiation of prenatal care were adjusted for in models. Healthy weight refers to a BMI of 18·5–24·9 kg/m². Obese refers to a BMI ≥30 kg/m². Women aged 25–29 years who were a healthy weight before pregnancy were the reference group. Error bars represent 95% CIs. OR=odds ratio.

Figure 4:. Joint association of maternal race or ethnicity and pre-pregnancy obesity status with risk of preterm birth among women aged younger than 20 years (A), 20–24 years (B),25–29 years (C), 30–34 years (D), 35–39 years (E), and 40 years or older (F)

Data are from the US National Vital Statistics System for 2016 and 2017. Parity, education level, smoking during pregnancy, previous history of preterm birth, marital status, infant sex, and timing of initiation of prenatal care were adjusted for in models. Healthy weight refers to a BMI of 18·5–24·9 kg/m². Obese refers to a BMI ≥30 kg/m². The reference group were non-Hispanic white women who had a healthy BMI before pregnancy. Error bars represent 95% CIs. OR=odds ratio.