Prospective observational study conducted on women with previous 1 or 2 CS candidate for elective CS. Ultrasonographic evaluation of lower uterine segment using 2D and 3D transabdominal and transvaginal then correlated to scar integrity assessed intraoperatively

Transabdominal ultrasound examination to obtain full obstetric ultrasound report including estimated gestational age, fetal lie, presentation, amniotic fluid volume, placental location with special focus on relation to the uterine scar.

The second step was to measure the thickness of the LUS using 2D ultrasound. View of the LUS was obtained in the mid-sagittal plane and the view was magnified looking for the thinnest area of the LUS. Also, the LUS was examined in a lateral view to detect any apparent rupture, ballooning or funneling. The measurement was taken with the cursors at the urine-Urinary bladder interface and the amniotic fluid -decidua interface after sufficient magnification and measurement was taken to the nearest tenth of millimeter. The numeric display was covered during the examination to avoid bias when performing the 3D measurement in the next step. Three measurements were taken and the least measurement was recorded.

After the entire thickness was measured by 2D ultrasound, 3 D measurement were taken in the mid-saggittal plan using the multiplanar display mode and then moving through the acquired 3-D volume in the saggittal plane till the thinnest area was seen and then measurements were taken.3-D volume dataset was obtained of the LUS by the same operator. The acquired volume was manipulated on the multiplanar display mode, searching for the thinnest part of the LUS avoiding obliquity.

Transvaginal ultrasound examination was done following the transabdominal one. The vaginal probe was inserted into the posterior vaginal fornix with the patient lying supine and the patient's knees are gently flexed and hips are slightly elevated with a pillow. Clear view of the LUS was obtained in the midsagittal plane visualizing the cervical canal to ensure that the view is midline one and avoiding obliquity.

On transvaginal examination the muscular layer of the LUS was clearly seen as hypoechoic line between the hyperechoic uterovesical fold and the decidualized endometrium and the chorioamniotic membranes. The scar area was magnified so the scar occupies at least 75% of the image to ensure consistent and accurate measurements. The thickness of the muscular layer of LUS was taken with the measuring caliber placed at the urinary bladder wall-myometrium interface and the myometrial/ chorioamniotic membrane interface .Three measurements of the LUS were taken, and again, the least measurement was recorded.

Again, 3D volume data set of the LUS was obtained .The acquired volume was manipulated on the multiplanar display looking for the thinnest part of the muscular layer. The thickness was measured to the nearest tenth of millimeter and recorded. The same operator has performed the transabdominal and transvaginal sonographic examination, and the surgeon performing the CS was always blinded of the sonographic findings.

The ultrasound examination to CS time interval varied from one to forty eight hours. During Cesarean delivery the LUS was defined as the part of the uterus below the uterovesical peritoneal reflection. After opening the visceral peritoneum and performing the bladder dissection, the LUS was assessed for integrity of the CS scar by the operating surgeon to avoid bias by the sonographic findings. Scar dehiscence was defined as the presence of either transparent LUS with visible contents, presence of well-circumscribed scar defect or presence of frank uterine rupture.

Inclusion Criteria:

• 37 - 40 weeks of gestational age
• singleton pregnancy,
• cephalic presentation,
• not in labor
• intact membranes
• with history of one or two previous CS

Exclusion Criteria:

• disorders of amniotic fluid,
• placenta previa
• history of other uterine surgery, e.g, myomectomy

women with prior 1 or 2 cesarean sections candidate for elective cesarean sewction