Statistical analysis
Image-quality scores were expressed as mean ± standard deviation (SD) or mean (95% confidence interval). Subgroups defined by trimester, time period, center, operator and cardiac position were compared using t-tests or ANOVA. When the ANOVA test was significant, a post-hoc test with Bonferroni correction was used for pairwise comparisons. When studied separately, quality criteria were expressed as a percentage for the three time periods and according to cardiac position.
To estimate intra-rater and inter-rater variability of the image-quality score, the intraclass correlation coefficient (ICC [95% confidence interval]) was estimated based on single-rating, absolute-agreement, 2-way mixed- (intra-rater) and random- (inter-rater) effects models. To estimate intra-rater and inter-rater variability of the individual (binary) quality criterion, Kappa coefficient was used.
All statistical analysis were carried out using IBM SPSS 20.0 for Windows software (IBM Inc., New York, USA). A value of p<0.05 was considered statistically significant.

Funding and conflicts of interest

The authors confirm that they have no conflicts of interest in this study.