Strengths and Limitations
The main strength of this study is that we have a considerably large
sample of Brazilian women from 17 centers in three regions, thus
covering possible internal population differences. In Brazil, previous
TVU performance analyses to predict PTB were from single-center studies18,23 with smaller samples. All cervical measurements
were performed by expert medical sonographers in tertiary reference
centers, along with checking of the ultrasound images to correct and
reinforce the pattern technique. We analyzed TVU using different
accuracy tests, different cutoff points and specific PTB subgroups for
gestational age.
The vaginal progesterone used for women with CL ≤30mm is a limitation in
our study because progesterone reduces the occurrence of PTB.
Nevertheless, in our prenatal clinical assistance, women with CL ≤25mm
are encouraged to use progesterone, so maintaining this intervention in
our sample allows the possibility to pragmatically infer the results to
medical practice. Unfortunately, we cannot identify if progesterone has
caused any reduction in PTB between women with CL 25-≤30mm, which could
have underestimated PTB incidence in this subgroup. Another limitation
is that some participating centers did not perform universal TVU
screening, which could introduce some selection bias in our sample and
the tendency to have a shorter CL. However, the mean CL identified was
very similar to other previous Brazilian studies16,24,25.