2.3 Statistical analysis
Statistical description was made by use of percentage for categorical
variables, and mean, along with standard deviation, for continuous
variables. The group difference was examined using
Chi-squared
test, variance analysis, or
Kruskal-Wallis rank test where appropriate, and the pairwise comparisons
were further checked.
The linear mixed models were used to test the fetal growth difference by
varied chorionicity and mode of conception. Both the overall
longitudinal change including trajectory curve and growth velocity
across the pregnancy and week-specific discrepancy were taken into
examination. For the purpose of this study, the difference was compared
between any two groups. For those ultrasound measurements which showed
statistically different either between SC-MCDA and SC-DCDA twins or
between SC-DCDA and ART-DCDA twins, week-specific comparison were
further conducted after adjusting for maternal age, height, weight,
gravidity, and parity5, 6. Moreover, all of the three
groups were in comparison with uncomplicated singletons, which delivered
in the same hospital at the same time.
The GAMLSS was applied to fit the growth curve of each biometric
parameter in relation to gestational age. The GAMLSS is highly flexible
as it is capable of modeling not only mean (or location) but also other
parameters (such as standard deviation, skewness and
kurtosis)10, 21. Box-Cox t (BCT) distribution was used
for modeling fetal biometric parameters as non-parametric cubic spline
functions of gestational age. Model selection was based on generalized
Akaike Information Criterion (GAIC), and the model with the smallest
value of GAIC was selected. Worm plots were used for visual inspection
of the fit of the smoothed curves. Centile curves for each biometric
parameter were constructed according to placental chorionicity and mode
of conception.
All analyses were performed with the Statistical Package for the Social
Sciences version 24 (SPSS Inc, Chicago, IL) and the GAMLSS and lme4
package for R statistical software (version 3.5.1). Significance was
defined as a 2-tail probability value of <0.05.