I am 20 weeks pregnant. I want to know if excessive laughing can cause any harm to my baby?
I'm not a doctor, but I would be fairly confident in saying that laughing will not harm your unborn child, as long as it does not harm you. There are extremely rare cases of non-pregnant adults dying from laughter, but not enough to worry about. I'd guess that in the long run, not laughing is worse for your health.
The effects you might be concerned about may be the feeling of breathlessness you experience when laughing and the vigorous shaking of your abdomen. I would suggest that both of these are comparable with moderate exercise, which is recommended during pregnancy:
If you were physically active before your pregnancy, you should be able to continue your activity in moderation. Don't try to exercise at your former level; instead, do what's most comfortable for you now. Low impact aerobics are encouraged versus high impact.
I can't find a reference for it, but I heard that a woman's body is careful to prioritise oxygen for the foetus during exercise.
Some studies have suggested that a mother's stress can have a negative impact on the unborn child, so perhaps laughter may even do them good.