There is a woman on the fertility forum that lifted something heavy and her placenta tore away from the uterine wall. The doctors told her that the lifting could have caused it. But I think that the above post is correct in that it is only a concern if you are straining and struggling to do it. If it is something that you are used to and comfortable with then I wouldnt worry.
If your plecnta detaches itself from the uterian wall, it's called Placental abruption. "Placental abruption is associated with certain complications for both you and your baby. If the placenta detaches completely from your uterine wall, it can deprive baby of oxygen and nutrients, interfering with fetal development, or even causing stillbirth. Women experiencing placental abruption are at increased risk for preterm delivery and newborn complications.
Unfortunately, the cause of placental abruption remains unknown. Certain factors do seem to contribute to the disorder, including abdominal trauma, premature rupture of the membranes, or smoking during pregnancy."
I have also been told that the placenta could start to detach itself from the strain...and quite frankly I don't want to find out if that is true. My doctor won't allow me to mow the lawn too (which drives me crazy) because he said the strain could put me into labor.
"All of the admonishments not to lift in pregnancy come from concerns over you hurting yourself, not you hurting the baby. During pregnancy, ligaments are soft and joints are less stable. It is easy to get an injury. In addition, because your center of gravity is shifted, it is easier for you to fall."
Found that on a website^ Also, I read that If it is something you are used to doing... Like lifting your dog.. If you have been lifting your dog since before you got pregnant than its nothing to worry about because your body is already accustomed to it.. Just like people who run marathons continue training when pregnant because their bodies are used to it. From what I read lifting is fine as long as it isnt uncomfortable and you arent straining or having a hard time breathing while doing it. I am 21 weeks and I just carried a bed frame and matress into the house and down 7 stairs yesterday with my 12 year old sister and had no problem. Just watch your back and make sure you are stable and not at risk for a fall.