Myometrial thickness according to uterine site, gestational age and prior cesarean delivery.

Objective: To evaluate the myometrial thickness (MT) of multiple uterine sites during all three trimesters of pregnancy in an attempt to identify anticipated myometrial thickness in each trimester for these sites.
Methods: In this prospective cross sectional study, MT was evaluated by ultrasound in women with viable singleton pregnancies. Women with suspected abruption, accreta, previa, fibroids, uterine and fetal anomalies, abnormal fluid volume, labor were excluded. MT was measured at the fundus, anterior wall (AW), posterior wall (PW), right and left side walls, lower uterine segment (LUS) and under the placenta. The cohort was divided to determine differences in MT at each site in each trimester (TRI). Differences in MT between sites were assessed.
Results: One hundred and seventy five women underwent ultrasound by a single observer. There were 25 1st, 100 2nd and 50 3rd TRI scans at mean (SD) gestations of 11.0 (1.9), 21.5 (3.6) and 34.1 (3.0) weeks, respectively. Women were 37% African American, 72% government insured, 39.4% nulliparous. A total of 13% of women had a prior cesarean. MT of the fundus was less than AW, PW, right and left side walls (p < 0.05 each). For all sites, MT was less in 2nd compared with 1st TRI (p < 0.0001 each). This was most evident with fundal and LUS MT (35% less each). There was no difference in MT between 2nd and 3rd TRI. Fundal, AW, left side wall, subplacental and LUS MT were greater in parous women (p < 0.05 each). With anterior placentation, AW MT was less than when the placenta was implanted elsewhere (6.6 vs 7.4 mm, p = 0.008). This was not found with fundal or posterior placentas. The LUS MT was not less with prior cesarean (6.1 vs 6.0 mm, p = 0.84).
Conclusion: Myometrial thickness of all uterine sites is less in 2nd and 3rd trimesters compared with the 1st trimester. Fundal MT is less than other upper segment MT in the 2nd and 3rd trimesters. LUS MT is not less with prior cesarean.