Sample size needed to achieve 80% power, in order to detect a difference in ppFEV1 decline during pregnancy comparing groups determined by practices, conditions or treatments
| Difference In Δ, A−B (%) | Prevalence Group A (%) | Sample size | |
| σ=7% | σ=12% | ||
| 5 | 10 | 180 | 510 |
| 5 | 25 | 84 | 244 |
| 5 | 50 | 62 | 184 |
We designed this study to detect any clinical meaningful negative impacts on maternal forced expiratory volume in 1 s (FEV1) occurring following any temporary discontinuation of modulators while pregnant. We assume that between 10% and 25% of women would discontinue modulator use during perhaps at least the first trimester of pregnancy, on the advice of their doctor. We further assume that the maximum tolerable difference in the change in FEV over pregnancy is 5%: that is, should women who remain on modulators have an average FEV drop of 3% over pregnancy, that women who discontinue modulators would have an average drop of no more than 8% ((−3%)−(−8%)=5%). Similarly, if women who remain on modulators have an average FEV increase of 4% over pregnancy, then women who discontinue modulators would have an average drop of no greater than 1% ((+4%)−(−1%)=5%). We also assume that the SD in the change in FEV1 % predicted is between 7% and 12%, based on previous studies of changes over the course of pregnancy.4 5
Δ=change post pregnancy versus prepregnancy. A, B=groups defined by baseline characteristics such as presence of cystic fibrosis-related diabetes or by modulator discontinuation over pregnancy. σ=SD deviation of change in per cent predicted forced expiratory volume in 1 s (ppFEV1), over pregnancy.