Statistical analysis
Based on the study by Thompson who found depression in 10% of the
unexposed and 16% of the exposed women at 4 months
postpartum,16 and including 2 women in the group
without PPH for every woman with it, we required 1542 women, including
514 exposed and 1028 unexposed, to have a power of 90%, with a
bilateral alpha equal to 5%.
We conducted a descriptive analysis of both groups — exposed and
unexposed — for the social, demographic, medical, and perinatal data.
Categorical variables were compared by chi-square (χ2)
tests (or Fisher’s exact test, as appropriate) and continuous variables
by Student’s t-test (or a Mann-Whitney test).
Prevalence rates and their 95%CIs for depression, anxiety, and PTSD
were calculated at 2, 6, and 12 months by χ2 tests (or
Fisher’s exact test, as appropriate).
The scores of each questionnaire were transformed in log form to obtain
a normal distribution. The results are presented as geometric means. The
geometric means were compared between the groups at each study time
point with Student’s t-test (or a Mann-Whitney test). A multivariate
analysis by a manual backward stepwise generalized linear model was
performed to study the association between exposure and the scores
obtained on each questionnaire. The adjusted geometric means were
compared between the groups. The results are also presented as crude and
adjusted odds ratios (ORs) with their 95%CIs.
Statistical significance was set at p<0.05. All statistical
analyses were performed with SAS software version 9, 2002-2010 (SAS
Institute Inc.).