Introduction

In Finland, in the 1990s, women giving birth became more overweight; i.e., the frequency of a prepregnancy BMI of >25 kg/m² rose from 18.8% in 1990 to 24.5% in 2000, and the frequency of obesity (BMI 30 kg/m²) rose from 7.5% to 11.0% . The same increasing trend in the prevalence of maternal obesity has been reported in other European countries and in the United States (1,2). At the same time, the percentage of pregnant women older than 35 years of age rose from 13.9% to 18.4% (3), and the mean maternal age at delivery rose from 29.1 years in 1990 to 29.9 years in 2000 (3), possibly contributing to the increased incidence of obesity during pregnancy.

Obesity is a known risk factor for several diseases and a major public health issue. It can lead to severe functional impairment and is associated with a considerable proportion of disability pensions (4). An overweight condition during pregnancy has been reported to increase morbidity, specifically hypertension (5,6,7), diabetes hypertension (5,6,7), gestational diabetes (2,6,7,8,9,10), and preeclampsia (5,6,7,10). Deliveries by obese women have been reported to be complicated by higher rates of labor induction (10,11), cesarean section (2,5,6,8,9,12,13,14,15,16), and dystocia resulting from macrosomia (2,6). A possible increase in the preterm birth rate is still controversial; some studies have shown an increased incidence (8), whereas others have not (6,10). Furthermore, a maternal overweight condition has been reported to be an independent risk factor associated with fetal death, showing a progressively increasing risk along with increasing maternal weight (8,12,17,18,19). The effect of maternal weight on perinatal deaths has been studied in only a few studies thus far, and the causes of increased mortality are still undefined (12).

Using a hospital database, we investigated all births from 1989 to 2001. The database includes data on maternal characteristics and pregnancy risk factors, complications, pregnancy outcome, and the neonatal period. We used the extensive database to control for possible confounding factors when calculating odds ratios (ORs)¹of various obstetric outcomes using logistic regression analysis in studying the effects of maternal overweight and obesity on pregnancy risks.

In Finland, the standard of maternity care is high. It is free of charge and is used by almost the entire (99.3% ) pregnant population (20). Maternity care starts early in pregnancy (3), and it consists of numerous visits: a minimum of 6 antenatal visits for normal multigravidas, 8 to 10 visits for primiparous women, and an average of 17 visits to maternity care units (3). Moreover, high technology is involved; there are very low rates of maternal and perinatal mortality (21), and there is a low incidence of mortality resulting from suboptimal care (22). Routine prenatal health care in maternity care units is provided by general practitioners and community midwives. However, treating obese pregnant women, with all of the known risks, is a challenge to maternity care. Our study hypothesis was that being even a minor degree overweight (BMI = 26 to 29 kg/m²) increases the risk of adverse pregnancy outcomes.

Research Methods and Procedures

We investigated the total population who gave birth at Kuopio University Hospital between January 1989 and December 2001, a total of 26,424 pregnancies. Information on maternal characteristics was based on data from self-administered questionnaires at 20 weeks of pregnancy, complemented by nurse interviews at visits to Kuopio University Hospital. The questionnaire consisted of more than 50 questions concerning marital status, employment, previous operations, illnesses, obstetric history, contraceptive use, smoking, alcohol consumption, and paternal characteristics. Information on pregnancy complications, pregnancy outcome, and the neonatal period was from clinical records. It was added to the database by the team who took care of delivery and neonatal care. The data were processed anonymously. They included 25,601 single pregnancies without major structural anomalies, of which 5268 involved overweight women (BMI = 26 to 70 kg/m²). Normal weight women (BMI 25 kg/m²) were used as a reference group. Two subgroups were created: overweight (pre-pregnancy BMI = 26 to 29 kg/m²) and obese (pre-pregnancy BMI 30 kg/m²). The women were measured at their first maternity care visit, in most cases before the 10th week of pregnancy, and BMI was calculated as kilograms per meter squared. Information on maternal weight was missing in 5.3% of the recorded pregnancies. Exclusion criteria were 1) multiple pregnancies (n = 548) and 2) major fetal structural anomalies (n = 275), because such pregnancies carry an unusually high risk of adverse outcome, and the effect of maternal weight on these pregnancies would be difficult to distinguish. The study population was ethnically homogeneous.

The following definitions were used: fetal death, intrauterine death of a fetus under 22 weeks of gestational age or 500 grams weight; perinatal death, intrauterine death after 22 weeks of gestation or 500 grams weight or during the first 7 days after birth; preterm birth, delivery before 37 weeks of gestation; prolonged pregnancy, delivery after 42 weeks of gestation; preeclampsia, repeated blood pressure measurement exceeding 149/90 mm Hg with proteinuria exceeding 0.5 g/d; maternal diabetes, insulin-treated diabetes during pregnancy; low birth weight, birth weight <2500 grams. Infants were considered small for gestational age when the age- and sex-adjusted birth weight was below the 10th percentile according to the normal tables for our population. Grand multiparity was defined having more than seven previous deliveries. The woman was considered a smoker when she smoked five cigarettes or more per day during pregnancy. We used a risk factor-based screening procedure for gestational diabetes followed by a 75-gram oral glucose tolerance test as the diagnostic test, using World Health Organization criteria, and up to one-third of all pregnant women, including all overweight and obese women, were tested. Alcohol use was recorded at interview; the pregnant woman either used or did not use alcohol before pregnancy and at 20 weeks of pregnancy. Illicit drug use was marginal in Kuopio in the 1990s. Low hemoglobin was defined as <100 g/liter in the third trimester of pregnancy. The pH limit used for fetal acidosis was 7.15 at birth. If a subject had two abnormalities, such as low birth weight and preterm delivery, each was considered an independent outcome, and the subject was included in both categories.

Statistical differences between the subjects and the reference group were evaluated using ² tests (dichotomous variables), and Fisher's exact test was applied when the minimal estimated expected value was less than five. A value of p < 0.05 was considered statistically significant. Continuous variables were analyzed using two-tailed, pooled Student's t tests. Possible confounding variables were identified from background data, obstetric risk factors, and health behavior. Multivariate analysis of significant or nearly significant effects (p < 0.1) of lifestyle variables concerned in this study was based on multiple logistic regression analysis (BMDP Statistical Software, Los Angeles, CA). Confidence intervals were evaluated at 95% (23,24).

Results

Of the women pregnant and giving birth at Kuopio University Hospital between January 1989 and December 2001, 13.2% were overweight (BMI = 26 to 29 kg/m²), and 7.3% were obese (BMI = 30 to 70 kg/m²) at the first maternity care visit. Compared with the normal weight women, the overweight and obese women were older: mean age in the normal weight women was 28.7 5.3 vs. 29.7 5.6 (SD) years (p < 0.001) in the overweight women and 29.9 5.5 years (p < 0.001) in the obese women. The youngest mother in this study was 14 years, and the oldest was 52 years.

Table 1 shows the distribution of maternal risk factors. The maternal risk profile differed statistically significantly (p < 0.001) between the study groups in nearly every aspect concerned: overweight and obese women were less often primiparous and more often multiparous. An unfavorable obstetric history, specifically previous pregnancy terminations, miscarriages, and instances of fetal demise, were more common in the overweight and obese women than in the normal weight women. Prior cesarean section had occurred more often among the obese and overweight women, and the pregnancy concerned was more often the second in 12 months or there was an interval of >6 years between the pregnancy concerned and prior pregnancy. Use of an intrauterine device as a contraceptive was more usual in the overweight and obese women; overweight and obese women smoked more often during pregnancy. Furthermore, the overweight and obese women more often had chronic illnesses, and the numbers of diabetic and hypertensive women were greater than in the normal weight women. Age <18 years was less common and age >35 years was more common in overweight and obese women. Only alcohol consumption during pregnancy did not vary between the study groups.

Table 1. - Reproductive risk factors among overweight and obese women vs. normal weight women.

Full table

Table 2 summarizes the frequencies of various pregnancy and delivery complications. The differences between study groups were statistically significant in nearly every aspect concerned. Preeclampsia and chorioamnionitis were more frequent in the overweight and obese women than in normal weight women. The amniotic fluid was more often found to be meconium-stained at delivery in obese and overweight women. On the other hand, no difference between the study groups was found in incidence of obstetric cholestasis. Low hemoglobin was less usual in overweight women than in the reference population.

Table 2. - Pregnancy and delivery characteristics in overweight and obese women vs. normal weight women.

Full table

The mean birth weight among newborns who were delivered at term, after 37 weeks of gestation, was 3489 587 grams in the normal weight women, 3618 662 grams in the overweight women, and 3616 736 grams in the obese women. Accordingly, the newborns of overweight and obese mothers weighed 156 and 128 grams more, respectively, than the newborns of normal weight mothers; these differences were statistically significant (p < 0.001). The highest birth weight was 5860 grams in the infants of normal weight mothers and 6020 grams in the infants of overweight/obese mothers. The incidence of clavicle fractures did not vary between the study groups, but Erb's paresis was a more common delivery complication in overweight/obese women than in normal weight women (p < 0.001).

Table 3 shows the adjusted ORs of adverse pregnancy outcomes in the three subgroups overweight, pooled (overweight and obese), and obese, after controlling for the obstetric risk factors investigated in this study in multiple logistic regression analysis. There was an increasing trend in the risk of adverse pregnancy outcome in the study groups: perinatal death rate, OR = 1.54 in the overweight study group, OR = 1.74 in the pooled study group, and OR = 2.19 in the obese study group; fetal death rate, ORs of 1.54, 1.79, and 2.35, respectively; low Apgar scores at 5 minutes of age, ORs of 1.54, 1.59, and 1.64, respectively; admission to a neonatal unit, ORs of 1.20, 1.26, and 1.38, respectively. The same trend was seen in the determinants of fetal asphyxia: abnormal fetal heart rate during delivery and low fetal venous pH at birth. The cesarean section rate also followed the same pattern, with ORs of 1.22, 1.37, and 1.68, respectively. However, no difference between the study groups was seen in the rates of instrumental delivery. Additionally, preterm delivery (before 37 weeks of gestation) and low birth weight (<2500 grams) seemed to be more common among overweight and obese mothers, but after adjusting for pregnancy risk factors, no statistically significant differences remained. On the other hand, the risk of having small-for-gestational age infants was higher in the normal weight women than in the overweight and obese women. Overall, the differences between unadjusted and adjusted ORs calculated in multivariate logistic regression analyses were small. In these analyses, the most predictive factors for perinatal death were primiparity [ p = 0.003, OR = 2.0, 95% confidence index (CI): 1.29 to 3.08] , previous miscarriage (p < 0.001, OR = 1.90, 95% CI: 1.27 to 2.84), previous stillbirth (p = 0.02, OR = 2.20, 95% CI: 1.29 to 3.08), and long interpregnancy interval (p = 0.01, OR = 1.87, 95% CI: 1.05 to 3.33).

Table 3. - Adjusted ORs of adverse pregnancy outcomes in various BMI groups.

Full table

Discussion

An overweight maternal condition exists in >20% of all pregnancies and represents a major public health issue because of the numerous known pregnancy risks and complications. In this study, we found an increasing trend in the risk of severe adverse obstetric outcomes, rising along with increasing maternal BMI. The risk of perinatal death was found to be high among overweight and obese women (OR = 1.54 to 2.19). The risk of fetal death was also found to be markedly increased (OR = 1.54 to 2.35), rising in a BMI-dependent manner. In addition, the risk of low Apgar scores (OR = 1.54 to 1.64) and incidence of the newborn being admitted to a neonatal intensive care unit (OR = 1.26 to 1.38) were high. The most predictive maternal background characteristics regarding perinatal mortality and fetal death were primiparity (OR = 2.0), previous miscarriage (OR = 1.90), previous stillbirth (OR = 2.20), and long interpregnancy interval (OR = 1.87). Maternal morbidity was also found to rise markedly when comparing overweight (BMI = 26 to 29 kg/m²) vs. obese (BMI 30 kg/m²) women: the incidence of maternal diabetes rose by 87% , that of hypertension by 77% , that of chronic diseases by 50% , and that of chorioamnionitis by 29% . At the same time, risky maternal health behavior, specifically smoking and alcohol consumption, and a history of previous pregnancy termination did not increase as much (by only 2.6% to 12% ). Maternal obesity even decreased the risk of anemia during pregnancy, which reflects good nutritional status.

The effect of an overweight condition on adverse pregnancy outcomes did not diminish in logistic regression analyses controlling for the numerous differences in maternal prepregnancy characteristics. We confirmed previously reported results of an overweight condition increasing the risks of maternal diabetes (5,6,7), preeclampsia (5,7,8,9,10), induced deliveries (10,11), prolonged pregnancies (12), and cesarean sections (2,5,6,8,9,12,13,14,15,16). Furthermore, we confirmed the results of previous studies regarding no excess risk of preterm birth (10,11) and a protective effect on fetal growth restriction (10).

The increasing trend toward obesity among pregnant women is costly. Because obesity is, at least in theory, a preventable threat to pregnancy, the excess fetal and perinatal mortality would be amenable to preventive measures. The costs arise from additional hospitalization of mothers as a result of chronic illnesses caused by obesity, an excess incidence of preeclampsia, extra pregnancy screening at maternity clinics, and an increased cesarean section rate, with increased risks of anesthesia complications, wound infections, and dehiscence owing to obesity (13,14,15,16). There is also the cost of treatment of infants at neonatal intensive care units as a result of maternal diabetes, birth asphyxia, and dystocia. Moreover, the consequences of being born to an obese mother with diabetes do not end in early childhood, and the offspring of diabetic mothers have been reported to be at an increased risk of diabetes and obesity; this risk does not seem to diminish with time but continues to adulthood (25). To gain more understanding of the mechanisms leading to the increase in fetal and perinatal deaths in obese mothers, further research is needed.

The prevalence of obesity increases in every age and social class, and it is a multifactorial trait, because both environmental and genetic factors are known to contribute to its development. It has been estimated that up to 40% to 80% of the variation in body weight is caused by genetic factors. In refining the phenotype, the mechanisms of body weight regulation, adiposity, and adipocyte metabolism, as well as feeding behavior, have to be taken into account (26,27,28). Whatever the mechanism, obesity is known to be associated with metabolic abnormalities, such as dyslipidemia, hypertension, insulin resistance, and endothelial dysfunction. During pregnancy, obesity, with its accompanying metabolic alterations, clearly is a risk factor for gestational diabetes, placental insufficiency, pregnancy-related hypertension, and preeclampsia (29,30), and even moderate weight loss would be protective against these complications.

The results of this study imply that maternity care comes too late to make a change in the pregnancies of overweight women. We suggest that obese women (BMI 30 kg/m²), at least, should be sent to maternity clinics for screening because of their high pregnancy risks and should be carefully monitored during delivery to lower the high risk of adverse pregnancy outcome. The results of a number of earlier studies showed no association between gestational weight gain and pregnancy complications (11,12,19,21), although excessive weight gain predisposes women to obesity-related problems after pregnancy. Thus, preventive measures should be taken with overweight teenagers before their first pregnancy, and delivery wards should have an essential role in identifying women at high risk in their next pregnancy as a result of obesity. Childbearing at a younger age should be encouraged, because it would reduce the number of overweight mothers. Even modest weight loss, aimed at keeping maternal weight under the limit of obesity (BMI 30 kg/m²), would bring substantial advantages to the obstetric outcome of these women.

Notes

¹ Nonstandard abbreviations: OR, odds ratio; CI, confidence index.

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Acknowledgments

There was no outside funding/support for this study.