There are some great websites showing photos of placentas which explain it all really well. The practice of rupturing membranes in labour is only conducted in hospitals and can have a deleterious effect, as Felicity said. It is common for it to be done at 3cm and with a woman who is confined to a bed this can often have the effect of cementing an OP baby into her pelvis without the cushion under it's head on which to move more easily. In hospitals the increase in pain from the cx at this point often lead to an epidural and then straight to caesarean. I've read that under normal physiological conditions rupture occurs spontaneously at around 8cm mostly but it's equally true that it doesn't happen at all for some babies and they are born in an intact membrane which is then gently peeled off their face. Magic, indeed! Nature has really good reasons for these parts of our bodies and they only increase our birthing potential if we leave them to do their work unhindered! There is also a small risk of injury to the baby from the hook used to rupture the membranes. I've seen some very nasty scarring on babies from this Rare but why risk it when combined with the other ill effects from this unnecessary procedure anyway?