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Challenges Posed by Weight Extremes

In this session of the workshop, presenters discussed the specific challenges associated with pregnancies at the extremes of the weight continuum—underweight and severe obesity.

CHALLENGES POSED BY UNDERWEIGHT

In 2023, slightly less than 3 percent of births in the United States were in women whose prepregnancy weight was classified as underweight, said Elizabeth Widen, University of Texas at Austin. These pregnancies tend to be concentrated in younger age groups, with a higher proportion in women under the age of 20. Widen also shared results from 2014 vital statistics records where underweight pregnancies were more common in Asian women, in women with lower education, and those on Medicaid. There are a number of reasons why a woman might be underweight prior to pregnancy, said Widen, including constitutional thinness, health conditions, or food insecurity. It is also possible that women may have lost weight via glucagon-like peptide 1 agonist (GLP-1) medications or bariatric surgery before becoming pregnant. Compared to pregnancies in women with normal weight, women with underweight tend to gain more weight and may gain more body fat during pregnancy, said Widen, but limited studies have examined gestational weight gain (GWG) and composition among underweight pregnancies with contemporary samples. She explained that when women gain weight in pregnancy, the fetus accounts for about 25 percent of the weight, with placenta and amniotic fluid accounting for another 10

percent. The remaining two-thirds of weight gain is from maternal supporting compartments and tissues, including the uterus, body water, and body fat.

Widen told participants about her study that used vital records from underweight pregnancies in Texas to examine predictors of low or high GWG. The 2009 Institute of Medicine guidelines recommend that women with underweight gain between 28 and 40 pounds, said Widen; the study found that more than 1 in 3 women gained less than the recommended amount, while more than 1 in 4 gained more than recommended. The proportion of inadequate GWG was higher among those who had preterm births, although Widen noted that these women had less time to gain weight. Researchers looked for predictors of both excessive GWG and inadequate GWG among underweight women who had full-term births. Compared to those gaining within recommendations, the risk of excessive GWG was higher with longer gestational age, Medicaid, and older maternal age, and lower for women who were foreign-born, had higher education levels, or had a prior pregnancy. The risk of inadequate GWG, compared to those gaining within recommendations, was higher for Black or Hispanic women compared to non-Hispanic White women, and lower for women with lower education. Inadequate weight gain was associated with negative perinatal outcomes (74 percent increased risk of preterm birth, 22 percent increased risk of neonatal intensive care unit [NICU] admission), and excessive GWG was associated with a 50 percent increased risk of gestational hypertension.

Widen and her colleagues also analyzed the data to determine whether infant mortality was related to GWG adequacy. They found that a higher proportion of infants not living at birth were born to women with inadequate GWG (0.41 percent), in comparison with women with adequate GWG (0.15 percent) and women with excessive GWG (0 percent). Widen cautioned that the data used in this study were limited to what was available on the birth record; this presents potential data-quality issues, and there was no information about the reason why women were underweight prior to pregnancy.

Widen identified some of the major research gaps in the area of underweight pregnancies. First, it is unclear how to support adequate GWG in women with underweight, what is driving inadequate GWG, and what interventions could make a difference, particularly given the heterogeneity in the drivers of underweight. Second, there is a need for research focused on specific issues within this area, including adolescent pregnancy, pregnancies among those with disordered eating, the effect of GLP-1s and weight-loss surgery, and the physiology of underweight pregnancies. Finally, said Widen, there is a need for research on the association between GWG among underweight pregnancies with morbidity and mortality.

CHALLENGES POSED BY SEVERE OBESITY

Pregnant women with severe obesity can face a number of complications in pregnancy and birth, said Michelle Kominiarek, Northwestern University Feinberg School of Medicine. Providers may face challenges in getting intravenous access, accurately measuring blood pressure, and transferring the patient if necessary. Pregnant women with obesity have a greater risk of cesarean delivery, which can lead to a greater risk of infection and hemorrhage. Providers and hospital staff may not have adequate training or equipment to appropriately care for pregnant women with severe obesity and may have biases that affect their care. There have been efforts to address these challenges through interventions at the time of birth, said Kominiarek, but most studies have not demonstrated any benefits (Kominiarek et al., 2024). With a lack of evidence-based interventions for caring for birthing women with obesity, one potential approach for reducing risks for these women and their children is minimizing weight gain during pregnancy.

Since 2009, studies have been conducted that have demonstrated the benefit of gaining less weight than the guidelines recommend or even losing weight during pregnancy. Kominiarek noted that losing weight during pregnancy goes against conventional wisdom, but there is evidence that a small amount of weight loss may improve outcomes for women with Class III obesity (Kominiarek et al., 2013). Specifically, weight loss may result in a lower risk of cesarean delivery; however, it is associated with a higher risk of small-for-gestational age (SGA).

Another area in which more research is needed is in pregnant patients who have lost weight with bariatric surgery. With increases in both prevalence of obesity and number of bariatric surgeries, she said, it is likely that an increasing number of people will be pregnant or preparing for pregnancy sometime after surgery. Weight loss—whether by surgery or other means—improves fertility, said Kominiarek, although it is commonly recommended that patients wait up to 2 years after surgery before conceiving. There are some older data on perinatal outcomes after weight loss surgery, but they tend to be small retrospective reviews, rely on self-reported outcomes, and have variability in control groups. One study of pregnancy data from Kaiser Permanente (Getahun et al., 2022) suggested that compared to the risks associated with severe obesity, bariatric surgery leads to lower risks of gestational diabetes, preeclampsia, cesarean delivery, large-for-gestational age, macrosomia, and admission to the NICU. The risks of outcomes such as stillbirth, surgical infection, placental abruption, and premature rupture of membranes remained the same, and the risks of SGA and postpartum hemorrhage were higher. Kominiarek noted that this study found that the increase and decrease in risks were the same regardless of the time interval between surgery and pregnancy.

Other studies have backed up the findings of an increased risk of SGA and decreased risk of LGA and macrosomia, she said. A scoping review looked at the effect of bariatric surgery on GWG and found that many patients after bariatric surgery still had obesity, and that patients were more likely to have excessive GWG (Deleus et al., 2024). Caring for patients after bariatric surgery requires adaptations, said Kominiarek. There may be metabolic changes that make screening for diabetes more challenging, and there is a need for close monitoring of weight changes. Questions remain about whether and how the 2009 GWG guidelines apply to patients who have had bariatric surgery, or whether the guidelines should be used as more of a screening measure than as a goal.

The recent increase in GLP-1 use also presents questions about the application of the GWG guidelines for these patients. The numbers are still very low, said Kominiarek, but the percentage of women who used a GLP-1 in the period shortly before pregnancy has increased dramatically and is expected to continue to rise. Like weight loss with bariatric surgery, fertility improves after weight loss with GLP-1s. Some patients may become pregnant unexpectedly, and little is known about the safety of GLP-1s around the time of pregnancy. Animal studies have found that use during pregnancy leads to early pregnancy loss, SGA, and skeletal anomalies. However, the limited evidence available in humans suggests that there is no increased risk for congenital malformations in pregnancies exposed to GLP-1s during the 90 days prior to conception through the 1st trimester. There are no data regarding any effect of GLP-1 “withdrawals” during pregnancy or on the relationship between GLP-1 use and subsequent GWG. Kominiarek noted that it is currently recommended that females discontinue use of some GLP-1s at least 2 months before a planned pregnancy. There is a need for further research on GWG and perinatal outcomes in this growing population, she said.