Morbid obesity before pregnancy is strongly associated with adverse obstetric and neonatal outcome.1 The appropriate gestational weight gain for these high-risk pregnancies is still unclear. In 2009, the Institute of Medicine released new recommendations regarding gestational weight gain and stated the optimal gestational weight gain for obese women (body mass index [BMI] 30 or higher; BMI is calculated as weight (kg)/[height (m)]2) to be 5–9 kg.2 The committee found insufficient evidence to stratify the recommendations in three obesity classes. The Institute of Medicine guidelines of a single standard of weight gain for all obese women have been called into question.3
A few earlier studies have examined the effects of variable gestational weight gain on obstetric outcome in morbidly obese women and there are indications that low weight gain or weight loss could improve both maternal and fetal outcome.4–6
Three studies used the 2009 Institute of Medicine-recommended weight gain as a reference2 Bodnar et al7 found that weight gain of 2.2 to less than 5 kg for obesity class III women was associated with a probability of less than 10% large-for-gestational-age and small-for-gestational-age births. Hinkle et al8 evaluated the risk for small for gestational age, large for gestational age, and macrosomia and suggested that a gestational weight gain below the Institute of Medicine guidelines may be associated with more favorable birth weight. In the third study, a decreased risk for large for gestational age and an increased risk for small for gestational age were found among obese women gaining below recommended gestational weight gain.9
The objective of this study was to estimate, in a large data set from the Swedish Medical Birth Registry, whether weight loss or low gestational weight gain in obesity class I–III is associated with adverse maternal and neonatal outcomes compared with gestational weight gain within the new Institute of Medicine recommendations.
MATERIALS AND METHODS
The study population consisted of 46,595 women with BMI 30 or higher with available data on gestational weight gain. These women gave births in Sweden January 1, 1993, through December 31, 2008. They were identified using the Swedish Medical Birth Registry.10 Medical and other data on almost all (99%) deliveries in Sweden are listed in the register, which also includes stillbirths after 28 weeks of gestation. It is based on copies of the standardized medical record forms completed at the maternity healthcare centers at the start of prenatal care, usually in gestational week 10–12, records from the delivery units, and the pediatric examination of the newborn. The system is identical throughout the country. A description and validation of the register content is available.10
Midwives recorded prepregnancy maternal weight and height on a standardized form at the first visit to the maternity healthcare center. Ninety percent of women who will give birth attend the antenatal clinic during the first trimester of their pregnancy.
Body mass index was calculated from maternal weight and height data. Only women with BMI 30 or higher were included in the study. Women were grouped into three obesity categories by BMI: obese class I (30–34.9), obese class II (35–39.9), and obese class III (40 or higher) according to World Health Organization.11
Gestational weight gain was defined as the difference between the maternal weight measured when the woman attended the delivery unit and the maternal prepregnancy weight recorded at the first visit to the maternity healthcare center.
The women were divided into four gestational weight gain categories according to the new Institute of Medicine recommendations: less than 0 kg (weight loss), 0–4.9 kg (low weight gain), 5–9 kg (recommended weight gain), and more than 9 kg (excessive weight gain).2
Outcomes studied were pre-eclampsia, the rate of cesarean delivery, instrumental vaginal delivery, small- and large-for-gestational-age neonate, fetal distress, and low Apgar score (less than 7 at 5 minutes).
Pre-eclampsia and fetal distress were registered in the Swedish Medical Birth Registry using the International Classification of Diseases (ICD). Pre-eclampsia was defined as ICD-9 codes 642.4–5 and ICD-10 codes O13–O15. Fetal distress was equivalent to ICD-9 codes 768.2–4 and ICD-10 codes P20.0, P20.1, and P20.9. Small-for-gestational-age newborns were defined as those with birth weight more than two standard deviations (SDs) below the mean birth weight for gestational age (sex- and parity-specific) according to a Swedish reference curve.12 Large for gestational-age newborns were those with a birth weight above two SDs. Gestational age was based on ultrasonographic estimation in the second trimester of pregnancy.
Maternal age (seven 5-year classes), parity (1–2+), and smoking in early pregnancy (unknown, no smoking, less than 10 cigarettes per day, 10 or more cigarettes per day) were thought to be potential confounding factors and were included as covariates in the adjusted analyses. The register information on these variables was obtained from the antenatal care center records.
Adjusted odds ratios (ORs) were determined using Mantel-Haenszel technique.13 This method gives a summary chi square from a series of two-by-two tables for various strata (eg, maternal age class, parity, and smoking) and the OR obtained from that OR. The 95% confidence interval) (95% CI) was estimated with Miettinen's method.14
We did not remove from the analysis women with a diagnosis of diabetes. Obesity is mainly associated with diabetes type 2 and diabetes type 2 will act as an intermediary in the effects of obesity.
The local ethics committee and the institutional review board at Faculty of Health Sciences, University of Linköping approved the study.
Maternal characteristics of the study population are presented in Table 1. The obese women were divided in three obesity classes: class I (n=32,991 [70.8%]), class II (n=10,068 [21.6%]), and class III (n=3,536 [7.6%]). The overall percentage of obese women gaining within the Institute of Medicine guidelines was 27.3. The proportion of women with excessive weight gain (more than 9 kg) was as follows: class I (59.8%), class II (48.4%), and class III (39.6%). There were no significant differences among the three obesity classes concerning the number of women gaining the recommended weight (5–9 kg). The prevalence of pre-eclampsia, large-for-gestational-age births, and small-for-gestational-age births among obese women (class III) gaining the recommended 5–9 kg was 11.5, 15.5, and 1.7%, respectively. The cesarean delivery rate in the same group was 28.4%.
Pregnancy, delivery, and neonatal outcome in three obesity classes and four weight gain categories are shown in Table 2 and Table 3. Obese women, irrespective of obesity class, who lose weight during pregnancy seem to have a decreased risk of cesarean delivery and large-for-gestational-age births compared with obese women gaining 5–9 kg and no significant increased risk for pre-eclampsia, excessive bleeding during delivery, instrumental delivery, low Apgar score, or fetal distress.
Small-for-gestational-age births are uncommon among obese women (1.2–3.8%) despite gestational weight gain. The risk for small for gestational age is twofold increased in obesity classes I and III among women who lose weight compared with women with recommended weight gain. The increased risk for small for gestational age disappears among women with low weight gain in obesity class III but persists in obesity class I. Obese (class III) women with high weight gain have no decreased risk of small for gestational age. Large-for-gestational-age births occur in one of five among the heaviest women with excessive weight gain. Obese women, class I–III with low weight gain (0–5 kg), do not differ markedly compared with obese women losing weight concerning both obstetric and neonatal outcome. The decreased risk of large for gestational age and cesarean delivery disappears and there is an increased risk for low Apgar score among women in obesity class II. Women in obesity class III with excessive weight gain do not have an increased risk for any of the evaluated outcomes, except for large for gestational age, compared with women in the same obesity class gaining within the recommendations. Obese women, class I and II, with excessive weight gain have an increased risk of pre-eclampsia, large for gestational age, cesarean delivery, instrumental delivery, excessive bleeding (class II), low Apgar score (class II), and fetal distress (class II). There is a slightly decreased risk for small for gestational age related to excessive weight gain most pronounced among women with the lowest BMIs.
This large population-based study showed that obese women (class II and III) who lose weight during pregnancy have a decreased risk of cesarean delivery and large-for-gestational-age births and no significant increased risk for pre-eclampsia, excessive bleeding during delivery, instrumental delivery, low Apgar score, or fetal distress compared with obese women (class II and III) gaining within the Institute of Medicine recommendations. There was a twofold increased risk for small-for-gestational-age births in this group, although small-for-gestational-age births are rare in obesity class II and III. The increased risk of small for gestational age disappears with low weight gain in obesity class III but persists in obesity class II. Large-for-gestational-age births occur in 13.2% of obese women, class II and III, gaining the recommended 5–9 kg. The prevalence decreases to 8.8% if they lose weight.
Obesity class II women with excessive weight gain have an increased risk for pre-eclampsia, cesarean delivery, instrumental delivery, bleeding, large for gestational age, and fetal distress. The risk for these adverse maternal and neonatal outcome variables is also increased in obesity class III, although not reaching statistical significance except for large for gestational age and cesarean delivery. Excessive weight gain did not decrease the risk for small for gestational age in obesity class III. A possible explanation could be intake of high-calorie, low-nutrient food.
Outcomes possible to compare between available studies stratified for obesity class and using the 2009 Institute of Medicine-recommended weight gain are large-for-gestational-age and small-for-gestational-age births, although the definition of large for gestational age differs. In the present study, large for gestational age is defined as two SDs above the mean birth weight for gestational age (sex- and parity-specific) compared with the three US studies using greater than the 90th percentile.7–9 In a study including 845 morbidly obese women (class III), the prevalence of large for gestational age was 15.3% and the risk associated with 50% gestational weight gain of Institute of Medicine recommendations (3.2 kg) was 0.8 (95% CI 0.7–0.9).7 In the present study, low weight gain (0–4.9 kg) in obesity class III did not significantly decrease the risk for large for gestational age. Hinkle et al found a decreased risk for large for gestational age (OR 0.46, 95% CI 0.30–0.71) in obesity class III and weight loss 0–4.9 kg.8 That is in accordance with results in this study, in which weight loss in obesity class III reduced the risk for large for gestational age (OR 0.64, 95% CI 0.46–0.90), and results from Park et al9 (OR 0.51, 95% CI 0.39–0.68).
Hinkle et al8 had two definitions of small for gestational age; one was two SDs below the sex- and race–ethnicity-specific mean birth weight, similar to this study. The overall small-for-gestational-age rate in obesity class II and III was 1.6% in this American data set, which is practically the same prevalence as in this study (1.9%). The risk for small for gestational age in obesity class III and weight loss of 0–4.9 kg was nonsignificant and an increased risk was first seen when women lost 5–13.6 kg (OR 1.58, 95% CI 1.08–2.32). In this Swedish data set, the risk for small for gestational age in obesity class III and weight loss was 2.34 (95% CI 1.15–4.76).
Comparisons of maternal obstetric outcomes stratified for obesity class and gestational weight gain categories based on the new Institute of Medicine recommendations are not possible because there are yet no available studies. In a recently published commentary on the Institute of Medicine guidelines, Rasmussen and coworkers15 stated that the scientific evidence was inadequate to provide specific guidelines for obesity class, although the guidelines for the first time considered the outcome of the mother.
Cesarean deliveries of morbidly obese patients are related to severe complications. In a recent study, 30% had wound complications in which 90% were wound disruptions.16 There is also an overall increased risk for anesthetic complications.17,18 Cesarean delivery is frequently used in obesity class II and III worldwide. The present study indicates that a low weight gain or weight loss could significantly reduce the risk of cesarean delivery with 34% (obesity class II) and 23% (obesity class III), respectively.
There is lack of data whether the risk for instrumental delivery is increased in obesity class II and III; in one study concerning obstetric outcome among morbidly obese patients, the risk in obesity class III was OR 1.34 (95% CI 1.16–1.56).1 This probable risk thus does not seem to be affected by weight loss or low gestational weight gain among morbidly obese parturients but increases with excessive weight gain based on findings in the present study. The pattern is practically the same for pre-eclampsia. The known increased risk for pre-eclampsia predicted by obesity increases with excessive gestational weight gain in all three obesity classes (66%, 58%, and 14%, respectively) but seems unaffected by gaining below the recommendations.
It must be kept in mind that the risk for unwanted obstetric and neonatal outcomes is high among morbidly obese women, despite weight loss or low gestational weight gain, compared with prevalence rates in the general population of pregnant women.
Excessive gestational weight gain substantially increases the risk of postpartum weight retention. A retrospective cohort study of 1,656 obese women showed that for each pound gained during pregnancy, there was a 0.4-pound increase above baseline weight at 1 year postpartum.19 If these women get pregnant again, they start with a higher prepregnancy BMI contributing to a worse obstetric outcome.
There are certain strengths and limitations of this study. An advantage of register studies is the large number of individuals, which gives high statistical power and makes it possible to demonstrate also weak effects on obstetric and neonatal outcome. Another advantage is the access to information on putative confounders. Prepregnancy weight and height were recorded in early pregnancy and therefore prospective regarding outcome. Recall bias was thus avoided. Maternal weight was measured at arrival to the delivery ward. It is possible that either obese women are weighed more often than normal weight women or that obese women refuse to be weighed more often than normal weight women.
In conclusion, it seems reasonably safe for obese women (class II and III) to lose weight because the risk decreases or remains unaffected for cesarean delivery, large for gestational age, pre-eclampsia, excessive postpartum bleeding, instrumental delivery, low Apgar score, and fetal distress. The twofold increased risk of small for gestational age in obesity class III and weight loss would give a prevalence of 3.7% small-for-gestational-age births, slightly above the overall prevalence of small-for-gestational-age births in Sweden (3.6%).20
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© 2011 by The American College of Obstetricians and Gynecologists.
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