I was told not to lift anything heavy by my doc when I started to spot early in the 1st tri. I'm not exactly sure what the reasoning was behind that. I stopped spotting and had a healthy pregnancy (after two early losses in a row in the previous 6 months). I was even told not to lift shopping bags.

I think mostly though it's to protect your back as everything is softening & stretching and lifting can cause damage. I think it's anything over 10kgs that's considered heavy.