From 40 weeks’ gestation, women classified as overweight have a double risk of stillbirth compared to women of normal weight, and the risk increased to almost four times higher for obese women. Women who were obese or severely obese had higher risk of almost all adverse pregnancy outcomes such as low Apgar score, stillbirth, transfer to neonatal care and instrumental delivery, compared to women of normal weight.
A recent study with data from the United States confirms the dose–response risk for stillbirth according to BMI and gestational week . Obesity has a causal relationship to various adverse pregnancy outcomes, such as stillbirth, and is one of the most important factors to focus on for prevention . Additionally, according to the risk of perinatal mortality, there is a curve-linear relationship, with higher risks in obese women from 39 week’s gestation, compared to women of normal weight (the longer the pregnancy progresses from 39 gestational weeks, the greater the risk for the obese). The underlying mechanisms for higher risk of stillbirth among overweight/obese women are still unclear. It is suggested that one cause for unexplained stillborn babies to women with higher BMI is that they are discretely small-for-gestational-age  and studies show higher risk of small-for-gestational-age by increasing BMI . In a Chinese population, obesity has been shown to be a risk factor for small-for-gestational-age babies (RR 2.66, CI 2.01–3.52) . The risk for giving birth to a small-for-gestational-age infant can also be linked to transgenerational transmission . An association is seen between placenta-mediated diseases, such as small-for-gestational-age and preeclampsia, and epigenetic factors that can be transferred to subsequent generations. The risk of having a small-for-gestational-age baby increases by almost three times if the mother has a small-for-gestational-age background herself . Insulin resistance, endothelial dysfunction, oxidative stress, lipotoxicity, inflammation, and infection are some possible mechanisms behind the higher risk for obstetrical complications for women who are overweight/obese. Obese women also have an increased risk of hypertension, preeclampsia, and impaired placental function, which can also be contributing factors for the higher risk of having a small-for-gestational-age baby or stillbirth . A high-fat diet may lead to dysfunction of placenta and higher risk of stillbirth, as seen in studies on primates and sheep [21, 22]. A reduction in uterine volume blood flow and increased placental inflammatory cytokines were seen among primates with intake of a high-fat diet.
In obstetric care the healthcare professionals weigh the pros and cons when inducing labor before the due date, with the aim of preserving the pregnancy, if possible. The advantages of inducing labor before the due date might save some babies’ lives, but the negative aspects are the medicalization of normal pregnancies and a risk of negative consequences for mother and baby in the short- and long-term perspective . Almost 50% of the women who were overweight/obese in our study were still pregnant when reaching 40 weeks’ gestation. Further, among women with stillbirth from 40 weeks’ gestation, a higher percentage were overweight or obese and additionally had two more risk factors for stillbirth, such as advanced maternal age and country of birth outside Sweden. This underlines the importance of considering inducing women having a risk pregnancy, if they pass their due date. In our data, according to number needed to treat among women with BMI from 25, compared to women of normal weight, a lower proportion of women must be induced from 40 weeks’ gestation to prevent one stillbirth (15 stillbirths inducing 10,368 women versus 19,783 women). If inducing all women who are overweight/obese from 40 weeks’ gestation, some babies in this study (approximately six), conducted in the capital of Sweden, could have been saved. Extrapolating these to numbers nationally, about 20 babies can be saved per year (Stockholm has 26% of all births in Sweden). However, we do not have information about the causes for death for the stillbirths in our study. It is possible that some of the babies who were stillborn from 40 weeks’ gestation could not be saved by an earlier induction of the delivery. Additionally, many diseases, for example diabetes, covariates with several other factors, such as ethnicity, which needs to be taken into account, as well as other confounding factors.
Our study confirms earlier research that obese and severe obese women have a higher risk of having severe obstetric and baby complications [24, 25]. Preventive methods in reducing overweight/obesity among young women is important [26, 27]. Beside public health policies, more resources must be allocated to youth centers , childcare centers and schools.
In the Stillbirth Lancet series, researchers identify overweight and obesity as important modifiable risk factors for stillbirth and stress that action is needed for prevention [6, 29]. Preferably, preventive methods should start at an early age to reverse the development that is taking place. If young women, for example adolescents in schools and at youth centers, are educated about health and pregnancy at an early stage, healthy diet and physical activity should be some of the areas to discuss. By informing women about the impact of these factors on pregnancy and overall health, and by giving support for healthy choices their risk of being overweight/obese when pregnant might be reduced. Additionally, if a woman shows interest in receiving further help in adhering to a healthier lifestyle, this woman can be identified and referred to other suitable professionals .
When investigating the 21 healthcare regions in Sweden, differences are seen in the guidelines for the management of women who are overweight and obese, when registered at a maternity clinic . The guidelines differ in when or whether a growth ultrasound should be conducted: 13 regions follow the same guidelines as if normal weight and in eight regions ultrasound is indicated if BMI > 35. Further, in seven regions an oral glucose tolerance test is indicated if BMI > 30 and in 13 regions if BMI > 35. The guidelines also differ in terms of whether a medical doctor or a dietician should be consulted and only 52 percent of the regional guidelines indicate that extra visits to the midwife is needed if the women are overweight/obese. It might be advantageous to have national, individualized care for women with BMI of 25 or above, as risk for negative obstetrical outcomes is linear according to BMI, and even modest increases in maternal BMI are associated with increased risk of stillbirth, perinatal and neonatal death . Further, the national guidelines should adapt to the clinical recommendations by FIGO (The International Federation of Gynaecology and Obstetrics) and health care policies should target women in a prepregnancy stage in primary care, as suggested in the recommendations by FIGO . Recently, NICE guidelines in the United Kingdom recommend labor induction from 41 weeks’ gestation and further recommend more research focusing on women with BMI of over 30 and women aged over 35 years which are groups of women who may be more likely to experience adverse outcomes if their pregnancy continues . This actual study clearly indicates that overweight and obese women need to be monitored extra closely during pregnancy and individually assessed for induction of labor versus close monitoring at term.
Strengths and limitations
A strength of this study is that the data are drawn from the population-based register, including a large number of data and almost all women giving birth in Stockholm. Further, there is a low percentage of missing values.
The study also has some limitations. In Sweden, there are regional differences in the prevalence of women with overweight and obesity. According to Chaparro et al. , Stockholm, which has been studied in this cohort, has the lowest prevalence. This would mean that the percentage of women who are overweight/obese is even higher in other parts of Sweden. The generalizability of the results is high for large cities in Sweden such as Stockholm, but less to other parts. However, this would mean that negative outcomes for women in rural areas could be even more pronounced.
We do not know the causes for stillbirth in this study. Additionally, we have no information on why the women who were obese were still pregnant around their due date. Some of the women could have been offered induction of labor but declined. Even if the risk of stillbirth increases by being overweight and prolonged pregnancy, the outcome is rare, and the woman’s choice is important in person-centered care.
The aim of the study was to identify sociodemographic factors and investigate outcomes related to BMI; we have not adjusted for potential confounders. When comparing obese women with women of normal weight, it is possible that other factors affect the outcomes among obese women. The comparing groups differ in sociodemographic factors, such as country of origin, educational level, previous stillbirth, tobacco use and diseases, factors that may have affected the outcomes.