Brief, sharp pain on both sides of your lower belly that comes and goes in a second is very commonly round ligament pain at 20 weeks. It happens as the ligaments supporting your growing uterus stretch, and it's one of the most common, harmless pains of mid-pregnancy.
Next Steps
Changing position slowly and avoiding sudden twists usually helps. Resting on your side when it happens, with a warm compress, can also ease it.
Health Tips
See your obstetrician the same day if the pain turns constant or severe, or comes with bleeding, fever, or fewer baby movements.