Very interesting reading everyone's comments! I have read that you need to be careful when lifting and that obviously, later in pregnancy it's not recommended. I am in a bit of a bind because my job involves lifting (paramedic), unfortunately people don't seem to want to move themselves when they're sick and injured (fair enough) so what do i do? There are many people in my job who have not miscarried and have had very healthy children and pregnancies. Given we do go on light duties (not sure when) but it's not until later definitely after 20weeks at the minimum i would think. I think it's all about common sense and safe handling procedures.
By the way - never heard constipation can cause a miscarriage - although I have heard that the straining can cause haemorrhoids - very unappealing!