Objective
Antenatal corticosteroids (ACS) decrease respiratory distress syndrome in singleton gestations. Twin data are less clear. Obesity and body mass index (BMI) also affect medication distribution volume. We evaluated whether maternal or neonatal cord betamethasone concentrations differed in twin gestations or obese patients.
Study Design
Participants receiving betamethasone in a randomized controlled trial of weekly ACS were identified. We analyzed maternal delivery and cord serum betamethasone concentrations comparing singletons with twins and obese (BMI ≥30 kg/m 2 ) with nonobese women.
Results
Fifty-five maternal and 45 cord blood samples were available. Unadjusted median maternal serum concentrations appeared paradoxically higher in both twin gestations and the obese. However, after controlling for confounders, there were no differences in betamethasone concentrations in maternal serum or cord blood between singletons and twins ( P = .61 vs P = .14) or nonobese and obese women ( P = .67 vs .12).
Conclusion
Maternal and umbilical cord blood serum betamethasone concentrations are not different in twin gestations or obese women.
Antenatal corticosteroids are administered to promote fetal pulmonary maturity in pregnancies at risk for preterm delivery between 24 and 34 weeks’ gestation. However, the dosing requirements in women with multiple gestations and maternal obesity have not been evaluated. Liggins and Howie described a dose of 12 mg of betamethasone given twice, 24 hours apart, in their seminal paper introducing this treatment. Although this regimen was chosen arbitrarily, it is the same regimen that many practitioners continue to use today and is endorsed by the American College of Obstetricians and Gynecologists and the National Institutes of Health Consensus Statement on antenatal corticosteroids.
Body composition, blood flow, and binding of plasma proteins are the principal factors that affect the volume of distribution of medication. During pregnancy blood volume increases by 40-45% on average, and greater increases are noted in twin gestations. This increased blood volume along with the decrease in oncotic pressure found in pregnancy can alter the volume of distribution of certain medications administered during pregnancy.
Antenatal corticosteroids have not reliably been shown to decrease the rate of respiratory distress syndrome (RDS) in twin gestations. It has been hypothesized that the more pronounced physiologic changes in twins compared with singletons may account for this observation.
Similarly, obesity can affect the volume of distribution of medication. An intramuscular injection of betamethasone soluspan contains both betamethasone phosphate, a short-acting, rapidly absorbed compound, and betamethasone acetate, a longer-acting compound with delayed absorption. The drug is then metabolized primarily in the liver but also in the kidney and surrounding tissue. The blood flow per gram of fat in normal-weight individuals is more than blood flow in the morbidly obese, and blood flow influences the volume of distribution. Although obesity does not seem to affect absorption, tissue distribution and drug elimination are changed.
This raises concerns as to whether the same concentration of betamethasone is available to the fetus of an obese mother compared with one who is normal weight. Therefore, we chose to study whether the number of fetuses (singleton or twin) or maternal obesity affects the concentration of betamethasone in maternal serum or fetal cord blood.
Materials and Methods
This is an analysis of biological fluids collected within a randomized, double-blinded, placebo-controlled clinical trial conducted by the Eunice Kennedy Shriver National Institute of Child Health and Human Development Maternal-Fetal Medicine Units Network that evaluated whether weekly injections of betamethasone decreased neonatal respiratory morbidity. Full details of the methods and study design have been previously reported.
All participants received 1 course of betamethasone prior to randomization. As part of the original study, maternal serum was collected at randomization and at delivery. Cord blood was also collected at delivery. These samples were centrifuged and frozen at –80°C for future analysis. The present analysis was confined to participants continuing to receive weekly betamethasone injections who delivered within 1 week of steroid administration.
All serum samples were analyzed in a single batch at Anapharm Inc (Québec, Canada). Determinations of betamethasone (Celestone; Schering-Plough, Kenilworth, NJ) concentrations were made using high-performance liquid chromatography with tandem mass spectrometry detection using an assay developed by Anaphram Inc. Specifically, the biological matrix used for the assay included human plasma (with EDTA) in which betamethasone was measured over a concentration range of 0.5–50 ng/mL. This method has sensitivity demonstrated at the lower limit of quantitation (0.500 ng/mL).
The quality control (QC) samples measured betamethasone at 4 levels, and they were within ±15% in accuracy and precision. Concentrations were calculated using the following softwares: MDS Sciex Analyst software, version 1.4.1. (Applied Biosystems/MDS Sciex, Ontario, Canada) and Thermo Electron Corp Watson LIMS software, version 7.0.0.01b (Inna Phase Corp, Philadelphia, PA).
The outcomes of interest for this analysis were the concentrations of betamethasone in the maternal serum and the cord blood. We also analyzed the ratio of maternal serum to cord blood betamethasone concentrations. Univariate analysis was used to evaluate betamethasone concentrations in nanograms per milliliter between singleton vs twin gestations and between women with prepregnancy body mass index (BMI) less than 30 kg/m 2 vs BMI 30 kg/m 2 or greater.
Continuous variables were analyzed using the Wilcoxon rank-sum test for maternal data and generalized estimating equations (GEE), a regression model that can be used to adjust for correlated observations for infant data. Multivariable linear regression analysis was used to examine the relationship between maternal serum betamethasone and obesity, plurality, gestational age at delivery, number of steroid study courses, and time between the last course of steroids and delivery.
GEE regression models were used to examine the relationship between cord blood betamethasone and these covariates. Log transformations of betamethasone levels were used in the regression analyses. Nominal statistical significance was set at a P < .05.
Results
There were 106 samples available for analysis (51 fetal [cord blood] and 55 maternal [delivery blood]). Six of the 51 cord blood samples were insufficient to assess betamethasone concentrations. Therefore, 45 cord blood and 55 maternal samples were analyzed comparing maternal and cord blood concentrations in singleton to twin pregnancies. All but 3 patients delivered secondary to spontaneous preterm labor with or without intact membranes.
Forty-five neonates delivered within 1 week of betamethasone administration; 30 were from singleton and 15 were from twin gestations. Of the 15, there were 6 twin pairs and 3 infants without samples from their cotwin.
In the univariate analysis, median maternal serum betamethasone concentrations were similar for singleton (4.4 ng/mL; interquartile range [IQR], 2.7–6.7) compared with twin (5.2 ng/mL; IQR, 3.1–7.8) gestations ( P = .49). Median cord blood betamethasone concentrations were higher in twin gestations, 4.4 ng/mL; IQR, 2.5–8.2, compared to singletons, 1.4 ng/mL; IQR, 1.0–3.0 ( P = .002).
Mothers pregnant with twins also received fewer courses of steroids than mothers of singletons, median 1 vs 3 courses, respectively ( P = .04). Figure 1 shows concentrations of betamethasone in cord blood for singletons and twins by days from last course. Concentrations of betamethasone decreased over time for both singletons and twins ( P < .0001).
