What does a uterine window on a previous cesarean section (C-section) scar feel like during pregnancy in a patient with a history of uterine surgery?

What Does a Uterine Window Feel Like During Pregnancy?

A uterine window on a previous cesarean scar feels like an extremely thin, bulging area of the anterior lower uterine segment where the uterine wall has become so attenuated that the placenta or fetal parts may be visible or palpable through it, even without abnormal placentation. 1

Physical Characteristics on Examination

Gross Palpation Features

• The affected area presents as a thinned and often bulging portion of the anterior lower uterine segment, typically overlying the bladder but may extend laterally toward the parametrium 1
• The uterine wall at this site becomes only a few millimeters thick, composed entirely of fibrotic scar tissue rather than normal myometrium 1
• On palpation during surgery or delivery, there is a gradual wedge-shaped transition from scar to normal myometrium at the edges of the bulge, distinguishing it from infiltrative placental invasion which shows irregular interfaces 1

Structural Composition

• The window consists of mature, thinned scar tissue with sometimes prominent branches of uterine arteries visible along the serosa 1
• This represents an incomplete disruption of the uterine wall at the site of prior cesarean delivery, technically termed uterine scar dehiscence (USD) 1
• The thin wall is prone to artifactual disruption during surgery due to its fragility, which must be distinguished from true placenta accreta spectrum invasion 1

Clinical Context and Risk Assessment

Prevalence of Scar Defects

• Cesarean section scar defects are remarkably common, with prevalence ranging between 24-88% on ultrasonography and 56-84% using contrast-enhanced sonohysterography 1
• Women with residual myometrial thickness (RMT) less than 2.5 mm are at substantially higher risk for developing clinically significant windows 2
• The mean RMT in asymptomatic women is approximately 5.39 ± 3.34 mm, which serves as a reference for normal healing 2

Risk Factors for Window Formation

• Multiple prior cesarean sections dramatically increase risk, with uterine rupture rates rising from 22 per 10,000 births after one cesarean to much higher rates with multiple procedures 1, 3
• Uterine position and surgical technique significantly influence scar healing, with single-layer closures showing less residual myometrial thickness than double-layer techniques 4
• Women with previous cesarean section have a 0.22-0.45% risk of complete uterine rupture in subsequent pregnancies, with significant geographic variation 3

Diagnostic Approach

Ultrasound Evaluation

• Transvaginal ultrasound is the primary diagnostic modality, with sensitivity of 77-97% and specificity of 96-98% for detecting abnormal placentation in high-risk patients 1
• The cut-off value of 4.15 mm for RMT provides 87.8% sensitivity and 71.3% specificity for predicting cesarean scar defects 2
• Doppler evaluation should be added if any abnormalities of placental tissue or placental-myometrial interface are detected on grayscale imaging 1

MRI Considerations

• MRI without contrast may be used when ultrasound is equivocal, particularly valuable for detecting parametrial invasion and surgical planning 1
• MRI provides superior soft-tissue contrast for detecting myometrial defects with intact serosal layers 3
• Gadolinium contrast remains controversial in pregnancy and should only be considered if benefits clearly outweigh fetal risks 1

Critical Clinical Pitfalls

Distinguishing Window from Placenta Accreta

• A uterine window can exist even in the absence of abnormal placentation, though the two conditions may coexist 1
• When placenta accreta spectrum involves an area of USD, care must be taken to distinguish surgical disruption from true invasion into adjacent tissues like perivesical fat 1
• The presence of placental villi overlying thinned scar without trophoblast infiltration into smooth muscle suggests window rather than true accreta 1

Implications for Subsequent Pregnancies

• Women with windows face increased risk of placenta previa (12 per 1000 deliveries) and placenta accreta in future pregnancies 1
• The absolute risk of placenta accreta increases dramatically: 3.3 per 10,000 with no prior cesarean, 12.9 per 10,000 after one cesarean, and 230 per 10,000 after five or more cesareans 1
• Corporeal uterine incision should be used for cesarean delivery to avoid surgical trauma to the lower uterine segment harboring the defect 1