The infants of obese pregnant women had a 55% higher adjusted perinatal death rate, compared with those of normal-weight pregnant women, but lower gestational age had a mediating effect, based on data from nearly 400,000 women-infant pairs.
“While some obesity-related causes of fetal death are known, the exact pathophysiology behind the effects of obesity on perinatal death are not completely understood,” Jeffrey N. Bone, MD, of the University of British Columbia, Vancouver, and colleagues wrote. Higher body mass index prior to pregnancy also is associated with preterm delivery, but the effect of gestational age on the association between BMI and infant mortality has not been well explored.
In a study published in PLOS ONE, the researchers reviewed data from nearly 400,000 women obtained through the British Columbia Perinatal Database Registry, which collects obstetric and neonatal data from hospital charts and from delivery records of home births. Births at less than 20 weeks’ gestation and late pregnancy terminations were excluded.
BMI was based on self-reported prepregnancy height and weight; of the 392,820 included women, 12.8% were classified as obese, 20.6% were overweight, 60.6% were normal weight, and 6.0% were underweight. Infants of women with higher BMI had a lower gestational age at delivery. Perinatal mortality occurred in 1,834 pregnancies (0.5%). In adjusted analysis, infant perinatal death was significantly more likely for obese women (adjusted odds ratio, 1.55) and overweight women (aOR, 1.22).
However, 63.1% of this association in obese women was mediated by gestational age of the infant at delivery, with aORs of 1.32 and 1.18 for natural indirect and natural direct effects, respectively, compared with that of normal-weight women. Similar, but lesser effects were noted for overweight women, with aORs of 1.11 and 1.10, respectively. “Direct effects were higher, and mediation was lower for stillbirth than for neonatal death, where the total effect was entirely indirect,” but the confidence intervals remained consistent with the primary analyses, the researchers noted.
The increased perinatal death rates of infants of obese and overweight women reflect data from previous studies, but the current study’s use of mediation analysis offers new insight on the mechanism by which perinatal death rates increase with higher maternal BMI, the researchers wrote.
The study findings were limited by several factors including the need to consider potential common risk factors for both perinatal death and early delivery that would be affected by maternal obesity, the researchers noted. Other limitations included the use of gestational age at stillbirth, which represents an approximation of fetal death in some cases, and the use of self-reports for prepregnancy maternal BMI.
However, the results were strengthened by the large, population-based design and information on potential confounding variables, and suggest that early gestational age at delivery may play a role in maternal obesity-related perinatal death risk.
“To better inform the pregnancy management in obese women, further studies should continue to disentangle the causal pathways under which obesity increases the risk of perinatal death, including, for example, gestational diabetes and other obesity-related pregnancy complications,” they concluded.
More testing and counseling are needed
The current study is important because obesity rates continue to increase in the reproductive-age population, Marissa Platner, MD, of Emory University, Atlanta, said in an interview. “Obesity has become a known risk factor for adverse pregnancy outcomes, specifically the risk of stillbirth and perinatal death. However, the authors correctly point out that the underlying cause of these perinatal deaths in women with obesity is unclear. Additionally, ACOG recently updated their clinical guidelines to recommend routine antenatal testing for women with obesity due to these increased rates of stillbirth.
“I was not surprised by these findings; similar to previous literature, the risks of perinatal death seem to have a dose-response relationship with increasing BMI. We know that women with prepregnancy obesity are also at higher risk of perinatal complications in the preterm period, which would increase the risk of perinatal death,” Dr. Platner said
“I think the take-home message for clinicians is twofold,” Dr. Platner said. First, “we need to take the updated antenatal testing guidelines from ACOG very seriously and implement these in our practices.” Second, “in the preconception or early antepartum period, these patients should be thoroughly counseled on the associated risks of pregnancy and discuss appropriate gestational weight gain guidelines and lifestyle modifications.”
However, “additional research is needed in a U.S. population with higher rates of obesity to determine the true effects of obesity on perinatal deaths and to further elucidate the underlying pathophysiology and disease processes that may lead to increased risk of both stillbirth and perinatal deaths,” Dr. Platner emphasized.
*This story was updated on March 23, 2022.
The study was supported by the Sick Kids Foundation and the Canadian Institute of Health Research. The researchers had no financial conflicts to disclose. Dr. Platner had no financial conflicts to disclose.