A healthy venous system is really important for both mother and baby during birth. My sister gave birth last year and was asked to do the valsalva maneuver (purple pushing). This is where the mother is asked to hold her breath for ten seconds and push at the same time. It's done in the second stage of birth and is widely practiced in hospitals.
I think for most people there are not much problems with this practice. With my sister though, the valsalva maneuver triggered fetal stress and she had bothersome swelling afterward. I'm also expecting a baby at the end of this year and have been reading about different practices that make birth easier. Apparently the valsalva maneuver can help with the delivery but it can also interfere with venous return. I read that sometimes the lack of venous return results in disruption of blood flow and the baby can be distressed because of that. It can also cause swelling.
For these reasons, I have spoken to my doctor that I do not want the valsalva maneuver during birth and he has agreed to that. I would suggest all expecting women to look into this before delivery, speak to your doctor about different options and make clear what you do and don't want. My sister was not aware of all this when she went into delivery and she was not asked whether she wanted it or not.