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Maternal pre-gravid body mass index and obstetric outcomes in twin gestations

Abstract

Objective:

The aim of this study is to evaluate the impact of maternal pre-gravid and/or first trimester overweight and obesity, and the adverse obstetrics outcome in twin pregnancies.

Study design:

This is a retrospective study of women who delivered viable twins after 23 weeks of gestation with available prepregnancy body mass index (BMI) and/or were at their earliest visit during the first trimester of pregnancy in the period 2007–2011. The patients were divided into four subgroups according to their BMI (underweight, normal weight, overweight and obese) according to the WHO classification and their outcomes were compared. Obstetrical outcomes of interest including gestational diabetes, gestational hypertension, preterm birth, antepartum hemorrhage, intrahepatic cholestasis of pregnancy, method of delivery and neonatal intensive care unit (NICU) admission were all studied and compared.

Result:

Electronic records of 1228 pregnant subjects who delivered twins were abstracted. Five hundred and four patients with twin gestations with available BMI were identified (underweight BMI<18.5% (n=22), normal weight BMI 18.5–24.9% (n=260), overweight 25–29.9% (n=114) and obese 30% (n=108)). Obstetric complications occurred more often in the overweight and obese groups as compared with the normal weight group. There was an increased risk of gestational diabetes in overweight and obese women (odds ratio (OR), 3.3; 95% confidence interval (CI) 1.52–7.3; P=0.001) and (OR, 3.2; 95% CI, 1.41–7.1; P=0.002), respectively. There was an increased risk of gestational hypertension in the obese group compared with the normal weight group (OR, 2.29; 95% CI, 1.1–4.7; P=0.02) but not in the overweight group (OR, 1.71; 95% CI, 0.8–3.6; P=0.1). In addition, an increased risk of very preterm delivery (<32 weeks) in the overweight group and obese groups was seen when compared with the normal weight group (OR, 2.2; 95% CI, 1.18–4.20; P=0.014 and OR, 2; 95% CI, 1.024–3.91; P=0.04, respectively). Increased rate of cesarean section in the obese group was seen when compared with the normal weight group (OR, 2; 95% CI, 1.2–3.4; P=0.006). Risks of antepartum hemorrhage, intrahepatic cholestasis and NICU admission were similar between the groups.

Conclusion:

In addition to the known obstetrics complications associated with twin gestations, the pregnancy outcomes in twins are further adversely influenced by increased maternal prepregnancy BMI.

Introduction

In 2011 ~4.6 million Canadian adults reported height and weight that classified them as obese (Source: Statistics Canada, Canadian Community Health Survey, 2003, 2005, 2007, 2008, 2009, 2010, 2011). The link between obesity and adverse birth outcomes is increasingly recognized as a major public health issue.1,2 This risk is expected to be higher in multiple pregnancies, especially with the increased rate of multiple pregnancies owing to the expanded use of assisted reproduction.3 When compared with singleton gestations; twins are at increased risk of adverse pregnancy outcomes and have significantly increased perinatal morbidity and mortality.4, 5, 6 These differences may be due to the alterations in maternal–fetal physiology and growth patterns in twin gestations.7,8 Suzuki et al.9 suggested that maternal obesity is a risk factor for very preterm delivery in dichorionic twin pregnancies. However, very few studies have examined the association between overweight and obesity, and pregnancy outcomes in twins.10, 11, 12, 13, 14

The aim of this study was to examine the prepregnancy weight groups, and to compare the obstetric outcomes of twin gestation.

Methods

This retrospective cohort study protocol was approved by the hospital research ethics board. Inclusion criteria included women delivering viable twins >23 weeks and with available prepregnancy or first trimester body mass index (BMI) for the period 2007–2011 at our institution. BMI (weight (kg) divided by height (m2)) was used to define maternal prepregnancy or first trimester weight groups. Electronic records of 1228 pregnant subjects were reviewed.

Gestational diabetes was diagnosed in pregnant women who failed routine screening in a 50-gram glucose challenge test at 24–28 weeks of gestation and had an abnormal 100 g oral glucose tolerance test, as per the American College of Obstetricians and Gynecologists criteria.15

The outcome of abnormal glucose challenge test per se was not looked at in our cohort and, similarly, the preexisting diabetes patients were excluded from the comparison.

Gestational hypertension was defined as a diastolic blood pressure of 90 mm Hg, based on the average of at least two measurements in the same arm. Proteinuria was defined as a urinary dipstick reading 2 plus.16 Patients with underlying hypertension were excluded from the comparison as the risk of developing gestational hypertension is higher as compared with normotensive women; also, we were looking specifically for cases who developed gestational hypertension.

Very preterm birth was defined as delivery before 32 weeks of gestation. All women had routine first trimester ultrasounds. The gestational age of the pregnancies was established by ultrasonographic examination of the fetal crown-rump length at 9–11 weeks of gestation in cases of spontaneous conception and embryo transfer dates when pregnancy was achieved by in vitro fertilization.

Antepartum haemorrhage is defined as bleeding from or into the genital tract, occurring from 24 weeks of pregnancy and before the birth of the baby.17

Intrahepatic cholestasis of pregnancy is an acquired form of cholestasis, which is observed in otherwise healthy pregnant women. It usually occurs in the second and third trimester of pregnancy, when serum concentrations of estrogen and progesterone reach their peak18,19 and is characterized by pruritus, elevated concentrations of bile salts, transaminases and rarely serum bilirubin.20, 21, 22 In our cohort, women who developed classical symptoms of intrahepatic cholestasis of pregnancy in the third trimester and/or abnormal-level transaminases (above 60 units l−1) and bile acids (above 20 μmol l−1) were diagnosed with cholestasis of pregnancy. The mode of delivery was also studied and compared among the groups.

Given the higher complications rate in monochorionic twins, we looked at the neonatal outcome and the number of cases that required admission to the neonatal intensive care unit in dichorionic twins only.

Statistical analysis

Descriptive statistics were used to describe the maternal characteristics. Pearson’s χ2 test was used for categorical variables.

Differences in outcome were determined using Pearson’s χ2-test for gestational diabetes, gestational hypertension, preterm birth, antepartum hemorrhage and intrahepatic cholestasis. A multivariate logistic regression model was created to examine preterm birth.

Given the small number of women in the underweight category, their characteristics and outcomes were described, but were not subjected to statistical analysis.

All analyses were completed using Stata/IC release 12.1 for Windows (Stata, College Station, TX, USA). For all analyses, a P value <0.05 was considered significant.

Results

Five hundred and four patients who had available prepregnancy BMI and/or were at their earliest visit during the first trimester of pregnancy were evaluated. The patients were divided into four groups by BMI according to the WHO classification23 (underweight BMI<18.5%, n=22; normal weight BMI 18.5–24.9%, n=260; overweight 25–29.9%, n=114; and obese 30%, n=108). Demographic information including maternal age, parity, chorionicity, pregestational hypertension, diabetes, hypothyroidism and smoking were recorded (Table 1).

Table 1 Comparison between overweight, obese and normal prepregnancy BMI by maternal charactersitics in twin pregnancy

The baseline characteristics were similar among the groups except for preexisting hypertension, which was higher in the obese group (Table 1).

Data on obstetric outcomes including gestational diabetes, gestational hypertension, preterm birth, antepartum hemorrhage and intrahepatic cholestasis were abstracted and compared (Table 2). The obese group had a higher rate of underlying essential hypertension, P value=0.001. In addition, there was an increased risk of gestational diabetes in the overweight and the obese groups compared with the normal weight group (OR, 3.3; 95% CI 1.52–7.3; P=0.001 and OR, 3.2; 95% CI, 1.41–7.1; P=0.002), respectively.

Table 2 Obstetrics outcome for overweight and obese prepregnancy BMI compared with normal BMI

After excluding the subjects with underlying hypertension, we found increased risk of gestational hypertension in the obese group compared with the normal weight group (OR 2.29; 95% CI, 1.1–4.7;P=0.02) but not in the overweight group (OR, 1.71; 95% CI, 0.8–3.6; P=0.1).

To account for the contribution of other factors associated with very preterm birth a multivariate model was created. Using a multivariate logistic regression model we studied the impact of prepregnancy BMI on very preterm delivery (birth occurring before 32 weeks) in all types of twins and after controlling for known confounding variables including hypertension, smoking, diabetes and chorionicity. In the multivariate model, overweight and obese prepregnancy BMI remained significantly associated with preterm birth when compared with the normal weight group (OR, 2.2; 95% CI, 1.18–4.20; P=0.014 and OR, 2; 95% CI, 1.024–3.91; P=0.04, respectively).

The rate of cesarean section was higher in the obese group compared with the normal weight group (OR, 2; 95% CI, 1.2–3.4; P=0.006); however, no difference was found between the overweight and normal weight groups.

There was no difference among the three groups with regard to neonatal intensive care unit admission in case of dichorionic twin pregnancies.

Risk of antepartum hemorrhage and intrahepatic cholestasis was similar between the three groups.

Discussion

Being overweight and obese are independent risk factors for antepartum complications when compared with being normal weight before twin pregnancy. Our observations are consistent with contemporary reports on the risk of medical complications in pregnancy among obese women with singleton pregnancies.24, 25, 26

Although the larger placental mass in twin gestation results in an increase in placental-derived steroid hormones that may predispose to glucose intolerance, studies have not consistently found an increase in gestational diabetes in pregnant women with twin gestation.12,13,27 Studies evaluating the risk of gestational diabetes in women carrying multiple pregnancies are conflicting, with some demonstrating an increase in the incidence of gestational diabetes28, 29, 30 and others demonstrating no difference when compared with singleton rates.12,31,32 Our findings were similar to Muktar et al.14 who reported an increased prevalence of diabetes, preeclampsia and chronic hypertension among prepregnant overweight and obese mothers with twin pregnancies when compared with their prepregnant underweight and normal weight counterparts.14,22

Reports of obese women with singleton gestations suggest an increased incidence of elective (induced vaginal or cesarean) preterm birth in nulliparous women. However, in another report spontaneous preterm labor in singleton pregnancies was found to be decreased among obese women.33 The average gestational age at delivery in twins is 35.2 weeks; therefore we aimed in our study to focus on very preterm births in twin gestations.34 Data are limited regarding the risk of preterm delivery in pregravid overweight or obese women with twin gestations. One study evaluated the maternal factors associated with very preterm delivery in dichorionic twin pregnancies, and found that complications were significantly increased among obese women.9

Pregnant women with twins have increased risk of third trimester bleeding from placenta previa35 or placental abruption.6 Some studies suggest an inverse relationship between pregravid maternal BMI and placental abruption. The mechanism by which obesity impacts the likelihood of placental abruption in twin pregnancies is not known.14 This risk for antepartum hemorrhage due to placenta previa or abruption was not found to be different between the groups in our study.

Gonzalez et al.36 evaluated the risk of intrahepatic cholestasis of pregnancy in twin pregnancies and found an increased risk compared with singleton pregnancies. Another report examined 263 twin pregnancies and reported the incidence of intrahepatic cholestasis to be 4.2% (11/263) without increased adverse perinatal outcomes.37 No studies have reported the risk of intrahepatic cholestasis in relation to maternal BMI. To our knowledge, our study is the first to address this issue in relation to prepregnancy BMI. We found no difference between the pregravid BMI groups of our women and intra-hepatic cholestasis.

The increased rate of cesarean section in the obese group in our data is supported by a metaanalysis by Chu et al.,38 who found that rates of cesarean birth increased by half among overweight women and by almost three times among severely obese women.

Conclusions

The limitations to our study include its retrospective nature and availability of recorded pregravid and or first trimester BMI data. Given the number of subjects we were not able to subcategorize obese subjects by BMI classes (that is, 1, 2 and 3). Our study had a multiethnic population and the information on race was inconsistently reported. Intrahepatic cholestasis of pregnancy is a relatively uncommon condition, and therefore we think that a bigger sample is required. We did not specify the indications of cesarean section and also did not look at the indications for neonatal intensive care unit admission among the three groups.

The strength of the study is that it is a relatively large study of obese and overweight women with twin gestation.

In summary, in addition to the known obstetrics complications associated with twin gestations, the pregnancy outcomes in twins are further adversely influenced by increased maternal prepregnancy BMI.

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Al-Obaidly, S., Parrish, J., Murphy, K. et al. Maternal pre-gravid body mass index and obstetric outcomes in twin gestations. J Perinatol 34, 425–428 (2014). https://doi.org/10.1038/jp.2014.29

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Keywords

  • body mass index
  • twins
  • gestational hypertension
  • gestational diabetes (GDM)
  • preterm delivery
  • neonatal intensive care unit (NICU)

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