During the first trimester visit, many medical practitioners will tell pregnant women not to lift more than 15 lbs until 6 weeks after the birth of their baby (maybe). I truly believe these practitioners are doing what they think is best, but unfortunately, they are missing the mark for the vast majority of women under their care.
The positive intention here is that there are a handful of women who should not be lifting more than 15 lbs due to an impeding medical condition. I also believe there are a handful of women who do not have sound movement patterns and lifting more than 15 lbs resulted in a preventable injury (prevention being the right coaching not necessarily the lifting itself). So, to reach the widest audience with medical advice, practitioners speak to the lowest common denominator and continue to recommend not lifting more than 15 lbs during pregnancy.