If you begin to suffer horrible, incapacitating constipation while you are pregnant, a gentle cleansing enema may be the best way to relieve it.
But don't let yourself get that far. I would suggest that you not let yourself go more than 48 hours without a good BM. If you are feeling that you are becoming constipated and unsweetened prune juice and Welches natural Concord grape juice don't seem to be stimulating a movement, then you might first try one or 2 glycerin suppositories. If they don't work within an hour or two, then it's time to turn to the enema bag.
A gentle enema consisting of a 2-3 pint saline solution (1 teaspoon of Sea Salt or non-iodized table salt per pint of bottled or filtered water), or a mild soapsuds solution using castile soap or a bar of Ivory soap stirred in warm water (103 degrees F at prep time - use a baby bath thermometer to verify) until it's milky, introduced slowly will usually provide immediate constipation relief.