Statistical analyses
Characteristics were compared between case and control mothers. Descriptive statistics of the study population were presented using means and standard deviations for normal distributed variables and median and interquartile range for skewed variables. Analysis was performed by Student t test and Mann-Whitney U test, respectively. Frequencies (proportions), were used for categorical variables and were compared by chi-square test. Spearman correlations was used to evaluate correlations between maternal age, and homocysteine, and TL. Multivariable logistic regression was used to compute crude and adjusted odds ratios (OR) per standard deviation (SD) decrease between the maternal T/S ratio with 95% confidence intervals (95% CI) and COD and VSD risk in offspring. The ORs were adjusted for maternal age, as known confounder for TL and COD (model 1). In a second model, we additionally adjusted for maternal body-mass-index, ethnicity, education and use of alcohol or smoking during pregnancy based on the characteristics of the two study populations and literature.(17, 18) Since homocysteine is an important oxidative stress marker, the analyses were also performed after adjustment for homocysteine concentrations.
Probability values ≤ 0.05 were considered statistically significant. All analyses were performed with R (R for Windows, version 3.5; R Core Team).
Funding The Bo Hjelt Foundation for Spina Bifida in memory of Madeleine Hjelt provided financial support for the conduct of the study and preparation of the manuscript. The funding source had no involvement in study design, data collection, analysis and interpretation of data, in the writing of the report nor in the decision to submit this article for publication.