As the unfortunate veteran of many miscarriages, I can tell you, when a baby is determined to stick, it takes something pretty extreme to knock him or her loose, and when a baby is not going to stick there is pretty much not too much that can be done to keep him or her around. The human body (all mammals in fact) have evolved remarkable adaptations to ensure the continuation of the species. A bike ride, a tennis match - this is nothing compared to the women who need to toil sunup to sundown in the fields, women who get pg in times of famine, etc. And yet, babies continue to get born.
Most doctors, even infertility specialists, if pushed will admit that most of the precautions they give women are for their own (the woman's) peace of mind. If you lie still in the office for an hour after an insemination and do no heavy lifting for 72 hours thereafter, if you keep your heartrate below 120 and do no exercise more strenuous than a brisk walk, then if you miscarry you are less likely to blame yourself.
Brina
"Truth goes through three stages: first it is ridiculed; then violently opposed; finally, it’s accepted as being self-evident." Schopenhauer