What is the reason for lower left abdominal pain in the early stages of pregnancy, around 40 days?

Patient's question:

At 40 days into early pregnancy, I'm experiencing mild pain in the lower left abdomen. The pain is worse at night and less noticeable during the day. Could it be an ectopic pregnancy?
Follow-up question: If it's not an ectopic pregnancy, what could be causing the pain?

Doctor's answer:

Hello, according to your situation, you may be experiencing threatened miscarriage. It is recommended that you actively go to the gynecology department of a hospital for examination to identify the cause, and then receive treatment.