You can have a miscarriage as early as your next period. That would be 2-3 weeks after conception. A miscarriage that early may signify that the fertilized egg was abnormal, or otherwise wouldn't have survived to mid or late pregnancy. Your body knows lots of things. Some women may not even know they had a miscarriage, but just think they just had a really painful and nasty period. A miscarriage "period" can be more painful than a normal one.