Apart from the prostaglandins in semen, which have been mentioned as a potential cause of difficulties if you have previously miscarried (as the prostaglandins can make the uterus contract as if you were in premature labor), there is another reason to be cautions about intercourse is miscarriage is a problem. Bacteria can be carried on the sperm, and if in the second trimester your cervix is short or already a little bit open, the bacteria might center the uterus and infect the fetus. The bacteria so introduced could lead to infectionof the membranes and the danger of their rupture, or they may bring about increased uterine contractions. These risk do not apply in normal pregnancy, where intercourse is not forbidden.