Purpose: To establish normal values for the lower uterine segment (LUS) thickness and to recognize factors that may affect these values.
Methods: In this cross-sectional study, we measured the LUS thickness in women between 11 and 41 weeks gestation. Women with placental abnormalities and fetal anomalies were excluded. Fetal weight, location of placental implantation, and cervical length were estimated. The LUS was examined to identify the thinnest zone and evaluated using the transvaginal approach with an empty urinary bladder. Full LUS thickness was measured with a caliper at the inner surface of the posterior bladder wall and another caliper at the interface between the amniotic fluid and the decidua. The myometrium was measured with the first caliper at the interface between the amniotic fluid and the deciduas and the second between the bladder wall and the myometrium. Normal values for the LUS were established throughout gestation, and correlations between the LUS gestational age, fetal weight, previous cesarean section, and other parameters were examined.
Results: Overall, 350 women were included in our study. Median parity was 1 (0-6) and the majority (62%, 218/350) of women have not had a previous cesarean section. The LUS thickness inversely correlated with gestational week (Pearson's r = -0.11, p < 0.03). When stratifying the linear regression model to women with and without a history of a previous cesarean section, it was demonstrated that gestational age was moderately correlated with LUS only in the second group of patients (Pearson's r = -0.16, p = 0.1 and Pearson's r = -0.47, p < 0.0001, respectively). During the post term period, patients who had had a previous cesarean section had significantly thinner LUS compared with patients who had not had a previous cesarean section (46 mm ± 20 versus 59 mm ± 20, p = 0.002, respectively). Neither parity nor the number of previous cesarean sections were found to correlate with the thickness of the LUS (p > 0.05). Multivariate linear regression (R(2) = 0.215, F(2, 63) = 9.021, p < 0.001) demonstrated that both gestational week (β = -0.13, p < 0.04) and cesarean section (β = -0.01, p < 0.0001) were independent factors associated with the thickness of the LUS; for every 1 week increase in gestational week, there was a 1.3 mm decrease in the total thickness of the LUS CONCLUSIONS: We have established reference ranges for LUS thickness throughout gestation. The LUS thickness decreases throughout gestation in singleton pregnancies and is significantly thinner in postterm period pregnant women with a previous cesarean section.
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