Women's prepregnancy underweight as a risk factor for preterm birth: a retrospective study

Abstract

Objective To investigate the distribution of known factors for preterm birth (PTB) by severity of maternal underweight; to investigate the risk adjusted relationship between severity of underweight and PTB and to assess if the relationship differed by gestational age. Design Retrospective cohort study. Setting State of California, USA. Methods Maternally linked hospital and birth certificate records of 950,356 California deliveries in 2007–2010 were analyzed. Singleton live births of women whose pre-pregnancy body mass index (BMI) was underweight (<18.5 kg/m2) or normal (18.50–24.99 kg/m2) were analyzed. Underweight BMI was further categorized as: severe (<16.00), moderate (16.00–16.99) or mild (17.00–18.49). PTB was grouped as 22–27, 28–31, 32–36 or <37 weeks (compared with 37–41 weeks). Adjusted multivariable Poisson regression modeling was used to estimate relative risk for PTB. Main outcome measures Risk of PTB. Results 72,686 (7.6%) women were underweight. Increasing severity of underweight was associated with increasing percent PTB: 7.8% (n=4421) in mild, 9.0% (n=1001) in moderate and 10.2% (475) in severe underweight. The adjusted relative risk of PTB also significantly increased: aRR=1.22 (95%CI: 1.19–1.26) in mild, aRR=1.41 (95%CI: 1.32–1.50) in moderate and aRR=1.61 (95%CI: 1.47–1.76) in severe underweight. These findings were similar in spontaneous PTB, medically indicated PTB, and the gestational age groupings. Conclusion Increasing severity of maternal pre-pregnancy underweight BMI was associated with increasing risk adjusted PTB at <37 weeks. This increasing risk was of similar magnitude in spontaneous and medically indicated births and in preterm delivery at 28–31 and at 32–36 weeks of gestation.

Gestational Weight Gain in Japanese Women With Favorable Perinatal Outcomes

“…In some recent studies, the lower pre-pregnancy BMI has been observed to contribute to the lower incidences of pregnancy-induced hypertension, gestational diabetes mellitus, macrosomia, cesarean delivery, postpartum hemorrhage and post-term delivery although an increasing severity of maternal underweight BMI had been reported to be associated with an increasing risk of premature delivery [12, 13]. The current result may be supported by these previous reports [12, 13]. However, there may be a serious limitation in this regard.…”

Significance of gestational weight gain in spontaneous onset of labor at term

“…As gestation progresses beyond term, perinatal morbidity and mortality further increase . In contrast, several studies have demonstrated an increased risk of spontaneous preterm labor in underweight pregnant women . In addition, low body mass index (BMI) is associated with a reduced risk of post‐term pregnancy .…”

The Role of Body, Mind, and Environment in Preterm Birth: Mind the Gap

“…Obese women also have an increased risk of developing preeclampsia, diabetes, and giving birth to infants with congenital anomalies, which are themselves risk factors for preterm birth . Micronutrient deficiencies or other associated factors such as smoking or illness have been suggested as possible explanations for the underweight‐preterm birth association …”

“…13,22 Maternal body mass index, both high and low, has also been linked to the risk of preterm birth. [26][27][28] The mechanisms behind these associations are largely unknown and are likely complex. Overweight and obesity may act via inflammatory or nutrition-related pathways, 26,27 and overeating itself may also be a stress response.…”

“…13 Micronutrient deficiencies or other associated factors such as smoking or illness have been suggested as possible explanations for the underweightpreterm birth association. 28 Another factor associated with the risk of preterm birth is maternal age. Both younger and older women have been shown to have an increased risk of preterm birth.…”