Gas exchange occurs in the membrane oxygenator (Figure 3) . Extracorporeal venous blood is exposed to fresh gas (or sweep gas) that oxygenates and removes carbon dioxide. Both oxygen uptake and carbon dioxide removal depend on the presence of a diffusion gradient as well as on the available surface area of the semipermeable membrane. What you need to know before your membrane sweep The stretch and sweep, or membrane sweep, done late in pregnancy can help you go into labour sooner. Here’s what you’ll want to know before you get it done. A membrane sweep is a way of bringing on labour when you're overdue. Your midwife or doctor may offer to do a membrane sweep after you have reached 40 weeks of pregnancy, though it will be more effective if done after 41 weeks. You may be offered a membrane sweep before other methods of induction, to get your labour going.