Conclusion
Women with CL≤25mm had a significant association with sPTB <34 weeks, which is an important clinical goal for preterm birth. Additionally, we found that the best cutoff points for all gestational ages outcomes (<37, <34, <32 and <28 weeks) are over 25mm. Considering the feasibility to perform CL measurement following a standard technique and the capability to detect almost one third of all sPTB <37 weeks, we suggest to use CL ≤30mm as the cutoff for cervical length to identify women at risk of sPTB. This is easier to remember and is very similar to the best cutoff point identified in our study. Thus, women with CL≤30mm should be recognized as at higher risk for PTB and those with CL≤25mm should be recognized and treated properly to reduce PTB<34 weeks.
It is important to highlight that although women with CL≤30mm are at higher risk for PTB, effective treatment for preventing PTB in women with 25–30mm CL are not available 26. These women should not be treated with progesterone, cervical pessaries, or cerclage because these treatments did not show clear benefits in reducing sPTB but should, however, receive a close antenatal care follow-up.
Considering the cutoff point where vaginal progesterone has demonstrated efficacy (25mm), the NNS of 248 to detect 18 women with CL ≤25mm is an acceptable number, which suggests the feasibility of implementing TVU for pregnant women in mid-trimester in settings like Brazil.
As most PTBs worldwide are concentrated in low- and middle-income countries, this analysis is important to describe specific results for our population and stimulate new studies in other similar settings focused on strategies to reduce PTB. In such countries, where economical resources are considerably limited, it is important to define with precision the best strategies to reduce costs while improving health care. Nowadays, the national antenatal care for Brazil has not adopted routine TVU at mid-trimester screening based on studies developed in high-income countries with lower rates of sPTB. The NNS estimated in our study creates an opportunity to review the Brazilian and other countries’ protocols to deal with the PTB prevention. The estimated NNS is considered low and acceptable and should underpin the implementation of the TVU as a mid-trimester screening test.