Main Findings
Our study identified a negative association between CL measured during the second trimester of pregnancy and the rate of sPTB. CL ≤31.7mm is an important risk factor for PTB at ≤37 weeks and CL ≤25mm is associated with extremely severe, severe, moderate and late PTB whereas CL of 25–30mm is associated with late PTB. This study also confirms previous observational studies that found low BMI, previous abortion, previous PTB, CL ≤30mm, funneling and sludge as predictors for PTB13–15.
The most relevant risk factor for PTB in a singleton pregnancy is a previous history of PTB; however, in nulliparous women this does not apply. We had almost half of the sPTB in nulliparous women and TVU is an important mean to identify nulliparous women at risk of PTB. In those women, except for BMI, the other important risk factors are directly connected to the second trimester TVU results. Thus, considering the higher incidence of sPTB in Brazil and globally 16, TVU is an important tool to routinely identify these women.
As a screening test for PTB, TVU did not present good performance to predict PTB at <37 weeks. This result agrees with previous studies that did not find high sensitivity or acceptable specificity to consider TVU as a screening test to predict late PTB17,18. Nevertheless, we can consider that TVU has a moderate prognostic performance to predict sPTB at <34 weeks and, moreover, has a good performance for predicting sPTB at <28 weeks, with a high sensitivity and acceptable specificity. The extremely severe and severe PTB correspond to only 5% of all premature deliveries but are responsible for most deaths associated with PTB 3.
There is an inverse correlation between long-term morbidity and adverse neurodevelopmental outcomes with gestational age at birth, which incurs higher medical costs and extrapolates this health problem to the economic sphere, generating a huge financial impact on the health system. The suggested NNS to identify a woman under real risk for an early preterm birth is very acceptable for a screening test. Thus, offering TVU as a screening test for women at risk of moderate and extreme sPTB would increase the reaching of optimal timing for antenatal corticosteroid administration 19 and allow preventive treatments for reducing sPTB as progesterone, cervical pessary or cerclage8,20.