Delayed cord clamping can lead to blood being pumped back into placenta?


I was talking to my doctor today and mentioned I wanted delayed cord clamping. He said the hospital tries to do it for 30-40 seconds with every birth (until it stops pulsing) and that they don’t want the blood to flow back into the placenta. (he said because the heart is beating at the same time as the placenta is pulsing so the blood can flow back 🤔)

I was thinking of doing delayed cord clamping for 3-5 minutes....I didn’t know that the blood could flow back...? How to prevent this issue so I could get the full 3-5 mins? Any moms who have done this before?