Placenta Previa & Uterine Blood Flow Simulator

Placental Position

Edge-to-os distance 20 mm (low-lying)

Lower-segment stretch (gestational age) 32%

Uterine contraction intensity 0%

Clinical Readout

Low-lying placenta

Edge 20 mm from internal os.

Hemorrhage risk0%

Intervillous flow

0 mL/min

Margin shear stress

0.0 Pa

Bleed rate

0 mL/min

Os coverage

How it works

Placenta previa is classified purely by geometry — the distance between the placental edge and the internal cervical os, measured by transvaginal ultrasound:

Complete/major:  os fully covered by placenta
Marginal:        edge reaches the os (0 mm) but does not cover it
Low-lying:       edge within 20 mm of the os
Normal:          edge > 20 mm from the os

As pregnancy advances the lower uterine segment thins and elongates (effacement) and, during contractions, the cervix and lower segment stretch further while the relatively inelastic placental tissue cannot follow. This mismatch produces shear at the placental margin, modeled here as:

τ_shear ∝ stretch × contraction × overlap_factor(distance)
overlap_factor = 1                 if edge ≤ 0 (covers os)
              = 1 − distance/20   if 0 < distance < 20 mm
              = 0                 if distance ≥ 20 mm

Placental perfusion itself follows a simplified Poiseuille relation for the spiral arteries feeding the intervillous space, Q = πΔPr⁴ / (8ηL), so flow rises with the fourth power of the (contraction-modulated) vessel radius. When shear stress exceeds the low-resistance venous sinuses' tolerance at the margin, low-pressure venous bleeding begins — classically painless, because it is a mechanical shearing of vessels, not a pressure-driven uterine event. This is why third-trimester bleeding with a normally positioned placenta ("painful," from abruption) and previa bleeding ("painless") are taught as opposite patterns in obstetrics.

Edge-to-os distance — repositions the placenta disc; negative values mean it overlaps the internal os.
Lower-segment stretch — simulates third-trimester elongation of the lower uterine segment as gestation advances.
Contraction intensity / pulse — Braxton-Hicks or labor contractions transiently increase stretch and shear at the margin.

Placenta Bleeding (shear) Spiral-artery inflow

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