Your body releases a lot of hormones that relaxes your muscles making it easier to overstretch so you should be careful and not try to lift anything heavier then your normal ability. I was told anything heavier then a two year you should be asking a second person to help anyway. I've also heard that there is the risk of detaching the placenta if you overstrain yourself as you use your middle section to lift large amount. Never heard of lifting being connected to a tangled umbilical cord tho.
What you do with your arms has absolutely nothing to do with what happens in your uterus. If your doctor or midwife restricts your activity you should certainly follow their instructions but it is not because of concerns about what the umbilical cord is doing. Raising your arms above your head or lifting heavy things is something most people do every day of their lives. It's okay.
Ultrasounds can show a tangled cord but most of the time people don't get ultrasounds right at the end of pregnancy. It's generally not a huge emergency anyway - I know lots of people who vaginally delivered babies with wrapped cords and not one baby had lasting complications.
Ur not supposed to lift over I think 20 lbs and not reach above your head.